Repeating decimal

Last updated

A repeating decimal or recurring decimal is a decimal representation of a number whose digits are eventually periodic (that is, after some place, the same sequence of digits is repeated forever); if this sequence consists only of zeros (that is if there is only a finite number of nonzero digits), the decimal is said to be terminating, and is not considered as repeating.

Contents

It can be shown that a number is rational if and only if its decimal representation is repeating or terminating. For example, the decimal representation of 1/3 becomes periodic just after the decimal point, repeating the single digit "3" forever, i.e. 0.333.... A more complicated example is 3227/555, whose decimal becomes periodic at the second digit following the decimal point and then repeats the sequence "144" forever, i.e. 5.8144144144.... Another example of this is 593/53, which becomes periodic after the decimal point, repeating the 13-digit pattern "1886792452830" forever, i.e. 11.18867924528301886792452830....

The infinitely repeated digit sequence is called the repetend or reptend. If the repetend is a zero, this decimal representation is called a terminating decimal rather than a repeating decimal, since the zeros can be omitted and the decimal terminates before these zeros. [1] Every terminating decimal representation can be written as a decimal fraction, a fraction whose denominator is a power of 10 (e.g. 1.585 = 1585/1000); it may also be written as a ratio of the form k/2n·5m (e.g. 1.585 = 317/23·52). However, every number with a terminating decimal representation also trivially has a second, alternative representation as a repeating decimal whose repetend is the digit 9. This is obtained by decreasing the final (rightmost) non-zero digit by one and appending a repetend of 9. Two examples of this are 1.000... = 0.999... and 1.585000... = 1.584999.... (This type of repeating decimal can be obtained by long division if one uses a modified form of the usual division algorithm. [2] )

Any number that cannot be expressed as a ratio of two integers is said to be irrational. Their decimal representation neither terminates nor infinitely repeats, but extends forever without repetition (see § Every rational number is either a terminating or repeating decimal). Examples of such irrational numbers are 2 and π. [3]

Background

Notation

There are several notational conventions for representing repeating decimals. None of them are accepted universally.

Different notations with examples
Fraction Vinculum Dots Parentheses Arc Ellipsis
1/90.10..10.(1)0.10.111...
1/3= 3/90.30..30.(3)0.30.333...
2/3= 6/90.60..60.(6)0.60.666...
9/11= 81/990.810..8.10.(81)0.810.8181...
7/12= 525/9000.5830.58.30.58(3)0.5830.58333...
1/7= 142857/9999990.1428570..14285.70.(142857)0.1428570.142857142857...
1/81= 12345679/9999999990.0123456790..01234567.90.(012345679)0.0123456790.012345679012345679...
22/7= 3142854/9999993.1428573..14285.73.(142857)3.1428573.142857142857...
593/53= 111886792452819/999999999999911.188679245283011..188679245283.011.(1886792452830)11.188679245283011.18867924528301886792452830...

In English, there are various ways to read repeating decimals aloud. For example, 1.234 may be read "one point two repeating three four", "one point two repeated three four", "one point two recurring three four", "one point two repetend three four" or "one point two into infinity three four". Likewise, 11.1886792452830 may be read "eleven point repeating one double eight six seven nine two four five two eight three zero", "eleven point repeated one double eight six seven nine two four five two eight three zero", "eleven point recurring one double eight six seven nine two four five two eight three zero" "eleven point repetend one double eight six seven nine two four five two eight three zero" or "eleven point into infinity one double eight six seven nine two four five two eight three zero".

Decimal expansion and recurrence sequence

In order to convert a rational number represented as a fraction into decimal form, one may use long division. For example, consider the rational number 5/74:

  0.0675    74 ) 5.00000         4.44           560           518            420            370             500

etc. Observe that at each step we have a remainder; the successive remainders displayed above are 56, 42, 50. When we arrive at 50 as the remainder, and bring down the "0", we find ourselves dividing 500 by 74, which is the same problem we began with. Therefore, the decimal repeats: 0.0675675675....

For any integer fraction A/B, the remainder at step k, for any positive integer k, is A × 10k (modulo B).

Every rational number is either a terminating or repeating decimal

For any given divisor, only finitely many different remainders can occur. In the example above, the 74 possible remainders are 0, 1, 2, ..., 73. If at any point in the division the remainder is 0, the expansion terminates at that point. Then the length of the repetend, also called "period", is defined to be 0.

If 0 never occurs as a remainder, then the division process continues forever, and eventually, a remainder must occur that has occurred before. The next step in the division will yield the same new digit in the quotient, and the same new remainder, as the previous time the remainder was the same. Therefore, the following division will repeat the same results. The repeating sequence of digits is called "repetend" which has a certain length greater than 0, also called "period". [4]

In base 10, a fraction has a repeating decimal if and only if in lowest terms, its denominator has any prime factors besides 2 or 5, or in other words, cannot be expressed as 2m5n, where m and n are non-negative integers.

Every repeating or terminating decimal is a rational number

Each repeating decimal number satisfies a linear equation with integer coefficients, and its unique solution is a rational number. In the example above, α = 5.8144144144... satisfies the equation

10000α − 10α= 58144.144144... − 58.144144...
9990α= 58086
Therefore, α= 58086/9990 = 3227/555

The process of how to find these integer coefficients is described below.

Formal proof

Given a repeating decimal where , , and are groups of digits, let , the number of digits of . Multiplying by separates the repeating and terminating groups:

If the decimals terminate (), the proof is complete. [5] For with digits, let where is a terminating group of digits. Then,

where denotes the i-th digit, and

Since , [6]

Since is the sum of an integer () and a rational number (), is also rational. [7]

Table of values

Thereby fraction is the unit fraction 1/n and 10 is the length of the (decimal) repetend.

The lengths 10(n) of the decimal repetends of 1/n, n = 1, 2, 3, ..., are:

0, 0, 1, 0, 0, 1, 6, 0, 1, 0, 2, 1, 6, 6, 1, 0, 16, 1, 18, 0, 6, 2, 22, 1, 0, 6, 3, 6, 28, 1, 15, 0, 2, 16, 6, 1, 3, 18, 6, 0, 5, 6, 21, 2, 1, 22, 46, 1, 42, 0, 16, 6, 13, 3, 2, 6, 18, 28, 58, 1, 60, 15, 6, 0, 6, 2, 33, 16, 22, 6, 35, 1, 8, 3, 1, 18, 6, 6, 13, 0, 9, 5, 41, 6, 16, 21, 28, 2, 44, 1, 6, 22, 15, 46, 18, 1, 96, 42, 2, 0... (sequence A051626 in the OEIS ).

For comparison, the lengths 2(n) of the binary repetends of the fractions 1/n, n = 1, 2, 3, ..., are:

0, 0, 2, 0, 4, 2, 3, 0, 6, 4, 10, 2, 12, 3, 4, 0, 8, 6, 18, 4, 6, 10, 11, 2, 20, 12, 18, 3, 28, 4, 5, 0, 10, 8, 12, 6, 36, 18, 12, 4, 20, 6, 14, 10, 12, 11, ... (=A007733[n], if n not a power of 2 else =0).

The decimal repetends of 1/n, n = 1, 2, 3, ..., are:

0, 0, 3, 0, 0, 6, 142857, 0, 1, 0, 09, 3, 076923, 714285, 6, 0, 0588235294117647, 5, 052631578947368421, 0, 047619, 45, 0434782608695652173913, 6, 0, 384615, 037, 571428, 0344827586206896551724137931, 3, 032258064516129, 0, 03, 2941176470588235, 285714... (sequence A036275 in the OEIS ).

The decimal repetend lengths of 1/p, p = 2, 3, 5, ... (nth prime), are:

0, 1, 0, 6, 2, 6, 16, 18, 22, 28, 15, 3, 5, 21, 46, 13, 58, 60, 33, 35, 8, 13, 41, 44, 96, 4, 34, 53, 108, 112, 42, 130, 8, 46, 148, 75, 78, 81, 166, 43, 178, 180, 95, 192, 98, 99, 30, 222, 113, 228, 232, 7, 30, 50, 256, 262, 268, 5, 69, 28, 141, 146, 153, 155, 312, 79... (sequence A002371 in the OEIS ).

The least primes p for which 1/p has decimal repetend length n, n = 1, 2, 3, ..., are:

3, 11, 37, 101, 41, 7, 239, 73, 333667, 9091, 21649, 9901, 53, 909091, 31, 17, 2071723, 19, 1111111111111111111, 3541, 43, 23, 11111111111111111111111, 99990001, 21401, 859, 757, 29, 3191, 211, 2791, 353, 67, 103, 71, 999999000001, 2028119, 909090909090909091, 900900900900990990990991, 1676321, 83, 127, 173... (sequence A007138 in the OEIS ).

The least primes p for which k/p has n different cycles (1 ≤ kp−1), n = 1, 2, 3, ..., are:

7, 3, 103, 53, 11, 79, 211, 41, 73, 281, 353, 37, 2393, 449, 3061, 1889, 137, 2467, 16189, 641, 3109, 4973, 11087, 1321, 101, 7151, 7669, 757, 38629, 1231, 49663, 12289, 859, 239, 27581, 9613, 18131, 13757, 33931... (sequence A054471 in the OEIS ).

Fractions with prime denominators

A fraction in lowest terms with a prime denominator other than 2 or 5 (i.e. coprime to 10) always produces a repeating decimal. The length of the repetend (period of the repeating decimal segment) of 1/p is equal to the order of 10 modulo p. If 10 is a primitive root modulo p, then the repetend length is equal to p  1; if not, then the repetend length is a factor of p  1. This result can be deduced from Fermat's little theorem, which states that 10p−1 ≡ 1 (mod p).

The base-10 digital root of the repetend of the reciprocal of any prime number greater than 5 is 9. [8]

If the repetend length of 1/p for prime p is equal to p  1 then the repetend, expressed as an integer, is called a cyclic number.

Cyclic numbers

Examples of fractions belonging to this group are:

The list can go on to include the fractions 1/109, 1/113, 1/131, 1/149, 1/167, 1/179, 1/181, 1/193, 1/223, 1/229, etc. (sequence A001913 in the OEIS ).

Every proper multiple of a cyclic number (that is, a multiple having the same number of digits) is a rotation:

The reason for the cyclic behavior is apparent from an arithmetic exercise of long division of 1/7: the sequential remainders are the cyclic sequence {1, 3, 2, 6, 4, 5}. See also the article 142,857 for more properties of this cyclic number.

A fraction which is cyclic thus has a recurring decimal of even length that divides into two sequences in nines' complement form. For example 1/7 starts '142' and is followed by '857' while 6/7 (by rotation) starts '857' followed by its nines' complement '142'.

The rotation of the repetend of a cyclic number always happens in such a way that each successive repetend is a bigger number than the previous one. In the succession above, for instance, we see that 0.142857... < 0.285714... < 0.428571... < 0.571428... < 0.714285... < 0.857142.... This, for cyclic fractions with long repetends, allows us to easily predict what the result of multiplying the fraction by any natural number n will be, as long as the repetend is known.

A proper prime is a prime p which ends in the digit 1 in base 10 and whose reciprocal in base 10 has a repetend with length p  1. In such primes, each digit 0, 1,..., 9 appears in the repeating sequence the same number of times as does each other digit (namely, p  1/10 times). They are: [9] :166

61, 131, 181, 461, 491, 541, 571, 701, 811, 821, 941, 971, 1021, 1051, 1091, 1171, 1181, 1291, 1301, 1349, 1381, 1531, 1571, 1621, 1741, 1811, 1829, 1861,... (sequence A073761 in the OEIS ).

A prime is a proper prime if and only if it is a full reptend prime and congruent to 1 mod 10.

If a prime p is both full reptend prime and safe prime, then 1/p will produce a stream of p  1 pseudo-random digits. Those primes are

7, 23, 47, 59, 167, 179, 263, 383, 503, 863, 887, 983, 1019, 1367, 1487, 1619, 1823, 2063... (sequence A000353 in the OEIS ).

Other reciprocals of primes

Some reciprocals of primes that do not generate cyclic numbers are:

(sequence A006559 in the OEIS )

The reason is that 3 is a divisor of 9, 11 is a divisor of 99, 41 is a divisor of 99999, etc. To find the period of 1/p, we can check whether the prime p divides some number 999...999 in which the number of digits divides p  1. Since the period is never greater than p  1, we can obtain this by calculating 10p−1 − 1/p. For example, for 11 we get

and then by inspection find the repetend 09 and period of 2.

Those reciprocals of primes can be associated with several sequences of repeating decimals. For example, the multiples of 1/13 can be divided into two sets, with different repetends. The first set is:

where the repetend of each fraction is a cyclic re-arrangement of 076923. The second set is:

where the repetend of each fraction is a cyclic re-arrangement of 153846.

In general, the set of proper multiples of reciprocals of a prime p consists of n subsets, each with repetend length k, where nk = p  1.

Totient rule

For an arbitrary integer n, the length L(n) of the decimal repetend of 1/n divides φ(n), where φ is the totient function. The length is equal to φ(n) if and only if 10 is a primitive root modulo n. [10]

In particular, it follows that L(p) = p − 1 if and only if p is a prime and 10 is a primitive root modulo p. Then, the decimal expansions of n/p for n = 1, 2, ..., p  1, all have period p  1 and differ only by a cyclic permutation. Such numbers p are called full repetend primes.

Reciprocals of composite integers coprime to 10

If p is a prime other than 2 or 5, the decimal representation of the fraction 1/p2 repeats:

1/49 = 0.020408163265306122448979591836734693877551.

The period (repetend length) L(49) must be a factor of λ(49) = 42, where λ(n) is known as the Carmichael function. This follows from Carmichael's theorem which states that if n is a positive integer then λ(n) is the smallest integer m such that

for every integer a that is coprime to n.

The period of 1/p2 is usually pTp, where Tp is the period of 1/p. There are three known primes for which this is not true, and for those the period of 1/p2 is the same as the period of 1/p because p2 divides 10p11. These three primes are 3, 487, and 56598313 (sequence A045616 in the OEIS ). [11]

Similarly, the period of 1/pk is usually pk–1Tp

If p and q are primes other than 2 or 5, the decimal representation of the fraction 1/pq repeats. An example is 1/119:

119 = 7 × 17
λ(7 × 17) = LCM(λ(7), λ(17)) = LCM(6, 16) = 48,

where LCM denotes the least common multiple.

The period T of 1/pq is a factor of λ(pq) and it happens to be 48 in this case:

1/119 = 0.008403361344537815126050420168067226890756302521.

The period T of 1/pq is LCM(Tp, Tq), where Tp is the period of 1/p and Tq is the period of 1/q.

If p, q, r, etc. are primes other than 2 or 5, and k, , m, etc. are positive integers, then

is a repeating decimal with a period of

where Tpk, Tq, Trm,... are respectively the period of the repeating decimals 1/pk, 1/q, 1/rm,... as defined above.

Reciprocals of integers not coprime to 10

An integer that is not coprime to 10 but has a prime factor other than 2 or 5 has a reciprocal that is eventually periodic, but with a non-repeating sequence of digits that precede the repeating part. The reciprocal can be expressed as:

where a and b are not both zero.

This fraction can also be expressed as:

if a > b, or as

if b > a, or as

if a = b.

The decimal has:

For example 1/28 = 0.03571428:

Converting repeating decimals to fractions

Given a repeating decimal, it is possible to calculate the fraction that produces it. For example:

(multiply each side of the above line by 10)
(subtract the 1st line from the 2nd)
(reduce to lowest terms)

Another example:

(move decimal to start of repetition = move by 1 place = multiply by 10)
(collate 2nd repetition here with 1st above = move by 2 places = multiply by 100)
(subtract to clear decimals)
(reduce to lowest terms)

A shortcut

The procedure below can be applied in particular if the repetend has n digits, all of which are 0 except the final one which is 1. For instance for n = 7:

So this particular repeating decimal corresponds to the fraction 1/10n  1, where the denominator is the number written as n 9s. Knowing just that, a general repeating decimal can be expressed as a fraction without having to solve an equation. For example, one could reason:

or

It is possible to get a general formula expressing a repeating decimal with an n-digit period (repetend length), beginning right after the decimal point, as a fraction:

More explicitly, one gets the following cases:

If the repeating decimal is between 0 and 1, and the repeating block is n digits long, first occurring right after the decimal point, then the fraction (not necessarily reduced) will be the integer number represented by the n-digit block divided by the one represented by n 9s. For example,

If the repeating decimal is as above, except that there are k (extra) digits 0 between the decimal point and the repeating n-digit block, then one can simply add k digits 0 after the n digits 9 of the denominator (and, as before, the fraction may subsequently be simplified). For example,

Any repeating decimal not of the form described above can be written as a sum of a terminating decimal and a repeating decimal of one of the two above types (actually the first type suffices, but that could require the terminating decimal to be negative). For example,

An even faster method is to ignore the decimal point completely and go like this

It follows that any repeating decimal with period n, and k digits after the decimal point that do not belong to the repeating part, can be written as a (not necessarily reduced) fraction whose denominator is (10n  1)10k.

Conversely the period of the repeating decimal of a fraction c/d will be (at most) the smallest number n such that 10n  1 is divisible by d.

For example, the fraction 2/7 has d = 7, and the smallest k that makes 10k  1 divisible by 7 is k = 6, because 999999 = 7 × 142857. The period of the fraction 2/7 is therefore 6.

In compressed form

The following picture suggests kind of compression of the above shortcut. Thereby represents the digits of the integer part of the decimal number (to the left of the decimal point), makes up the string of digits of the preperiod and its length, and being the string of repeated digits (the period) with length which is nonzero.

Formation rule CodeCogsEqn(4).gif
Formation rule

In the generated fraction, the digit will be repeated times, and the digit will be repeated times.

Note that in the absence of an integer part in the decimal, will be represented by zero, which being to the left of the other digits, will not affect the final result, and may be omitted in the calculation of the generating function.

Examples:

The symbol in the examples above denotes the absence of digits of part in the decimal, and therefore and a corresponding absence in the generated fraction.

Repeating decimals as infinite series

A repeating decimal can also be expressed as an infinite series. That is, a repeating decimal can be regarded as the sum of an infinite number of rational numbers. To take the simplest example,

The above series is a geometric series with the first term as 1/10 and the common factor 1/10. Because the absolute value of the common factor is less than 1, we can say that the geometric series converges and find the exact value in the form of a fraction by using the following formula where a is the first term of the series and r is the common factor.

Similarly,

Multiplication and cyclic permutation

The cyclic behavior of repeating decimals in multiplication also leads to the construction of integers which are cyclically permuted when multiplied by certain numbers. For example, 102564 × 4 = 410256. 102564 is the repetend of 4/39 and 410256 the repetend of 16/39.

Other properties of repetend lengths

Various properties of repetend lengths (periods) are given by Mitchell [12] and Dickson. [13]

for some m, but
then for c  0 we have

For some other properties of repetends, see also. [14]

Extension to other bases

Various features of repeating decimals extend to the representation of numbers in all other integer bases, not just base 10:

combined with a consecutive set of digits
with r := |b|, dr := d1 + r − 1 and 0 ∈ D, then a terminating sequence is obviously equivalent to the same sequence with non-terminating repeating part consisting of the digit 0. If the base is positive, then there exists an order homomorphism from the lexicographical order of the right-sided infinite strings over the alphabet D into some closed interval of the reals, which maps the strings 0.A1A2...Andb and 0.A1A2...(An+1)d1 with AiD and Andb to the same real number – and there are no other duplicate images. In the decimal system, for example, there is 0.9 = 1.0 = 1; in the balanced ternary system there is 0.1 = 1.T = 1/2.
represents the fraction

For example, in duodecimal, 1/2 = 0.6, 1/3 = 0.4, 1/4 = 0.3 and 1/6 = 0.2 all terminate; 1/5 = 0.2497 repeats with period length 4, in contrast with the equivalent decimal expansion of 0.2; 1/7 = 0.186A35 has period 6 in duodecimal, just as it does in decimal.

If b is an integer base and k is an integer, then

For example 1/7 in duodecimal:

which is 0.186A35base12. 10base12 is 12base10, 102base12 is 144base10, 21base12 is 25base10, A5base12 is 125base10.

Algorithm for positive bases

For a rational 0 < p/q < 1 (and base bN>1) there is the following algorithm producing the repetend together with its length:

functionb_adic(b,p,q)// b ≥ 2; 0 < p < qdigits="0123...";// up to the digit with value b–1begins="";// the string of digitspos=0;// all places are right to the radix pointwhilenotdefined(occurs[p])dooccurs[p]=pos;// the position of the place with remainder pbp=b*p;z=floor(bp/q);// index z of digit within: 0 ≤ z ≤ b-1p=b*pz*q;// 0 ≤ p < qifp=0thenL=0;ifnotz=0thens=s.substring(digits,z,1)endifreturn(s);endifs=s.substring(digits,z,1);// append the character of the digitpos+=1;endwhileL=pos-occurs[p];// the length of the repetend (being < q)// mark the digits of the repetend by a vinculum:forifromoccurs[p]topos-1dosubstring(s,i,1)=overline(substring(s,i,1));endforreturn(s);endfunction

The first highlighted line calculates the digit z.

The subsequent line calculates the new remainder p′ of the division modulo the denominator q. As a consequence of the floor function floor we have

thus

and

Because all these remainders p are non-negative integers less than q, there can be only a finite number of them with the consequence that they must recur in the while loop. Such a recurrence is detected by the associative array occurs. The new digit z is formed in the yellow line, where p is the only non-constant. The length L of the repetend equals the number of the remainders (see also section Every rational number is either a terminating or repeating decimal).

Applications to cryptography

Repeating decimals (also called decimal sequences) have found cryptographic and error-correction coding applications. [15] In these applications repeating decimals to base 2 are generally used which gives rise to binary sequences. The maximum length binary sequence for 1/p (when 2 is a primitive root of p) is given by: [16]

These sequences of period p  1 have an autocorrelation function that has a negative peak of −1 for shift of p  1/2. The randomness of these sequences has been examined by diehard tests. [17]

See also

Notes

  1. Courant, R. and Robbins, H. What Is Mathematics?: An Elementary Approach to Ideas and Methods, 2nd ed. Oxford, England: Oxford University Press, 1996: p. 67.
  2. Beswick, Kim (2004), "Why Does 0.999... = 1?: A Perennial Question and Number Sense", Australian Mathematics Teacher, 60 (4): 7–9
  3. "Lambert's Original Proof that $\pi$ is irrational". Mathematics Stack Exchange. Retrieved 2023-12-19.
  4. For a base b and a divisor n, in terms of group theory this length divides
    (with modular arithmetic ≡ 1 mod n) which divides the Carmichael function
    which again divides Euler's totient function φ(n).
  5. Vuorinen, Aapeli. "Rational numbers have repeating decimal expansions". Aapeli Vuorinen. Retrieved 2023-12-23.
  6. "The Sets of Repeating Decimals". www.sjsu.edu. Archived from the original on 23 December 2023. Retrieved 2023-12-23.
  7. RoRi (2016-03-01). "Prove that every repeating decimal represents a rational number". Stumbling Robot. Archived from the original on 23 December 2023. Retrieved 2023-12-23.
  8. Gray, Alexander J. (March 2000). "Digital roots and reciprocals of primes". Mathematical Gazette . 84 (499): 86. doi:10.2307/3621484. JSTOR   3621484. S2CID   125834304. For primes greater than 5, all the digital roots appear to have the same value, 9. We can confirm this if...
  9. Dickson, L. E., History of the Theory of Numbers, Volume 1, Chelsea Publishing Co., 1952.
  10. William E. Heal. Some Properties of Repetends. Annals of Mathematics, Vol. 3, No. 4 (Aug., 1887), pp. 97–103
  11. Albert H. Beiler, Recreations in the Theory of Numbers, p. 79
  12. Mitchell, Douglas W., "A nonlinear random number generator with known, long cycle length", Cryptologia 17, January 1993, pp. 5562.
  13. Dickson, Leonard E., History of the Theory of Numbers, Vol. I, Chelsea Publ. Co., 1952 (orig. 1918), pp. 164173.
  14. Armstrong, N. J., and Armstrong, R. J., "Some properties of repetends", Mathematical Gazette 87, November 2003, pp. 437–443.
  15. Kak, Subhash, Chatterjee, A. "On decimal sequences". IEEE Transactions on Information Theory, vol. IT-27, pp. 647–652, September 1981.
  16. Kak, Subhash, "Encryption and error-correction using d-sequences". IEEE Transactios on Computers, vol. C-34, pp. 803–809, 1985.
  17. Bellamy, J. "Randomness of D sequences via diehard testing". 2013. arXiv : 1312.3618

Related Research Articles

<span class="mw-page-title-main">Decimal</span> Number in base-10 numeral system

The decimal numeral system is the standard system for denoting integer and non-integer numbers. It is the extension to non-integer numbers of the Hindu–Arabic numeral system. The way of denoting numbers in the decimal system is often referred to as decimal notation.

<span class="texhtml mvar" style="font-style:italic;">e</span> (mathematical constant) Constant value used in mathematics

The number e is a mathematical constant approximately equal to 2.71828 that can be characterized in many ways. It is the base of the natural logarithm function. It is the limit of as n tends to infinity, an expression that arises in the computation of compound interest. It is the value at 1 of the (natural) exponential function, commonly denoted It is also the sum of the infinite series There are various other characterizations; see § Definitions and § Representations.

In mathematics, a continued fraction is an expression obtained through an iterative process of representing a number as the sum of its integer part and the reciprocal of another number, then writing this other number as the sum of its integer part and another reciprocal, and so on. In a finite continued fraction, the iteration/recursion is terminated after finitely many steps by using an integer in lieu of another continued fraction. In contrast, an infinite continued fraction is an infinite expression. In either case, all integers in the sequence, other than the first, must be positive. The integers are called the coefficients or terms of the continued fraction.

<i>p</i>-adic number Number system extending the rational numbers

In number theory, given a prime number p, the p-adic numbers form an extension of the rational numbers which is distinct from the real numbers, though with some similar properties; p-adic numbers can be written in a form similar to decimals, but with digits based on a prime number p rather than ten, and extending to the left rather than to the right.

In recreational mathematics, a repunit is a number like 11, 111, or 1111 that contains only the digit 1 — a more specific type of repdigit. The term stands for "repeated unit" and was coined in 1966 by Albert H. Beiler in his book Recreations in the Theory of Numbers.

In arithmetic, long division is a standard division algorithm suitable for dividing multi-digit Hindu-Arabic numerals that is simple enough to perform by hand. It breaks down a division problem into a series of easier steps.

<span class="mw-page-title-main">Square root of 2</span> Unique positive real number which when multiplied by itself gives 2

The square root of 2 is a real number that, when multiplied by itself or squared, equals the number 2. It may be written in mathematics as or . It is an algebraic number, and therefore not a transcendental number. Technically, it should be called the principal square root of 2, to distinguish it from the negative number with the same property.

<span class="mw-page-title-main">Power of two</span> Two raised to an integer power

A power of two is a number of the form 2n where n is an integer, that is, the result of exponentiation with number two as the base and integer n as the exponent.

In mathematics, a periodic sequence is a sequence for which the same terms are repeated over and over:

<span class="mw-page-title-main">Minkowski's question-mark function</span> Function with unusual fractal properties

In mathematics, Minkowski's question-mark function, denoted ?(x), is a function with unusual fractal properties, defined by Hermann Minkowski in 1904. It maps quadratic irrational numbers to rational numbers on the unit interval, via an expression relating the continued fraction expansions of the quadratics to the binary expansions of the rationals, given by Arnaud Denjoy in 1938. It also maps rational numbers to dyadic rationals, as can be seen by a recursive definition closely related to the Stern–Brocot tree.

A cyclic number is an integer for which cyclic permutations of the digits are successive integer multiples of the number. The most widely known is the six-digit number 142857, whose first six integer multiples are

A decimal representation of a non-negative real number r is its expression as a sequence of symbols consisting of decimal digits traditionally written with a single separator: Here . is the decimal separator, k is a nonnegative integer, and are digits, which are symbols representing integers in the range 0, ..., 9.

In mathematics, Midy's theorem, named after French mathematician E. Midy, is a statement about the decimal expansion of fractions a/p where p is a prime and a/p has a repeating decimal expansion with an even period. If the period of the decimal representation of a/p is 2n, so that

271 is the natural number after 270 and before 272.

In mathematics, an infinite periodic continued fraction is a continued fraction that can be placed in the form

<span class="mw-page-title-main">Rational number</span> Quotient of two integers

In mathematics, a rational number is a number that can be expressed as the quotient or fraction of two integers, a numerator p and a non-zero denominator q. For example, is a rational number, as is every integer. The set of all rational numbers, also referred to as "the rationals", the field of rationals or the field of rational numbers is usually denoted by boldface Q, or blackboard bold

<span class="mw-page-title-main">Real number</span> Number representing a continuous quantity

In mathematics, a real number is a number that can be used to measure a continuous one-dimensional quantity such as a distance, duration or temperature. Here, continuous means that pairs of values can have arbitrarily small differences. Every real number can be almost uniquely represented by an infinite decimal expansion.

<span class="mw-page-title-main">Irrational number</span> Number that is not a ratio of integers

In mathematics, the irrational numbers are all the real numbers that are not rational numbers. That is, irrational numbers cannot be expressed as the ratio of two integers. When the ratio of lengths of two line segments is an irrational number, the line segments are also described as being incommensurable, meaning that they share no "measure" in common, that is, there is no length, no matter how short, that could be used to express the lengths of both of the two given segments as integer multiples of itself.

The digits of some specific integers permute or shift cyclically when they are multiplied by a number n. Examples are:

<span class="mw-page-title-main">Reciprocals of primes</span> Sequence of numbers

The reciprocals of prime numbers have been of interest to mathematicians for various reasons. They do not have a finite sum, as Leonhard Euler proved in 1737.