Freshman's dream

Last updated
An illustration of the Freshman's dream in two dimensions. Each side of the square is X+Y in length. The area of the square is the sum of the area of the yellow region (=X ), the area of the green region (=Y ), and the area of the two white regions (=2xXxY). Freshman's Dream.svg
An illustration of the Freshman's dream in two dimensions. Each side of the square is X+Y in length. The area of the square is the sum of the area of the yellow region (=X ), the area of the green region (=Y ), and the area of the two white regions (=2×X×Y).

The freshman's dream is a name given to the erroneous equation , where is a real number (usually a positive integer greater than 1) and are non-zero real numbers. Beginning students commonly make this error in computing the power of a sum of real numbers, falsely assuming powers distribute over sums. [1] [2] When n = 2, it is easy to see why this is incorrect: (x + y)2 can be correctly computed as x2 + 2xy + y2 using distributivity (commonly known by students in the United States as the FOIL method). For larger positive integer values of n, the correct result is given by the binomial theorem.

Contents

The name "freshman's dream" also sometimes refers to the theorem that says that for a prime number p, if x and y are members of a commutative ring of characteristic p, then (x + y)p = xp + yp. In this more exotic type of arithmetic, the "mistake" actually gives the correct result, since p divides all the binomial coefficients apart from the first and the last, making all the intermediate terms equal to zero.

The identity is also actually true in the context of tropical geometry, where multiplication is replaced with addition, and addition is replaced with minimum. [3]

Examples

Prime characteristic

When is a prime number and and are members of a commutative ring of characteristic , then . This can be seen by examining the prime factors of the binomial coefficients: the nth binomial coefficient is

The numerator is p factorial(!), which is divisible by p. However, when 0 < n < p, both n! and (pn)! are coprime with p since all the factors are less than p and p is prime. Since a binomial coefficient is always an integer, the nth binomial coefficient is divisible by p and hence equal to 0 in the ring. We are left with the zeroth and pth coefficients, which both equal 1, yielding the desired equation.

Thus in characteristic p the freshman's dream is a valid identity. This result demonstrates that exponentiation by p produces an endomorphism, known as the Frobenius endomorphism of the ring.

The demand that the characteristic p be a prime number is central to the truth of the freshman's dream. A related theorem states that if p is prime then (x + 1)pxp + 1 in the polynomial ring . This theorem is a key fact in modern primality testing. [4]

History and alternate names

In 1938, Harold Willard Gleason published a poem titled «"Dark and Bloody Ground---" (The Freshman's Dream)» in The New York Sun on September 6, which was subsequently reprinted in various other newspapers and magazines. It consists of 2 stanzas, each containing 8 lines with alternating indentation; it has an ABCB rhyming scheme. Words and phrases that hint that it might be related to this concept include: "Algebra", "Wild corollaries twine", "surds", "of plus and minus sign", "binomial", "quadratic", "parenthesis", "exponents", "in terms of x and y", "remove the brackets, radicals, and do so with discretion", and "factor cubes". [5]

The history of the term "freshman's dream" is somewhat unclear. In a 1940 article on modular fields, Saunders Mac Lane quotes Stephen Kleene's remark that a knowledge of (a + b)2 = a2 + b2 in a field of characteristic 2 would corrupt freshman students of algebra. This may be the first connection between "freshman" and binomial expansion in fields of positive characteristic. [6] Since then, authors of undergraduate algebra texts took note of the common error. The first actual attestation of the phrase "freshman's dream" seems to be in Hungerford's graduate algebra textbook (1974), where he states that the name is "due to" Vincent O. McBrien. [7] Alternative terms include "freshman exponentiation", used in Fraleigh (1998). [8] The term "freshman's dream" itself, in non-mathematical contexts, is recorded since the 19th century. [9]

Since the expansion of (x + y)n is correctly given by the binomial theorem, the freshman's dream is also known as the "child's binomial theorem" [4] or "schoolboy binomial theorem".

See also

Related Research Articles

In mathematics, a field F is algebraically closed if every non-constant polynomial in F[x] has a root in F. In other words, a field is algebraically closed if the fundamental theorem of algebra holds for it.

<span class="mw-page-title-main">Binomial coefficient</span> Number of subsets of a given size

In mathematics, the binomial coefficients are the positive integers that occur as coefficients in the binomial theorem. Commonly, a binomial coefficient is indexed by a pair of integers nk ≥ 0 and is written It is the coefficient of the xk term in the polynomial expansion of the binomial power (1 + x)n; this coefficient can be computed by the multiplicative formula

In elementary algebra, the binomial theorem describes the algebraic expansion of powers of a binomial. According to the theorem, the power expands into a polynomial with terms of the form , where the exponents and are nonnegative integers satisfying and the coefficient of each term is a specific positive integer depending on and . For example, for ,

In mathematics, more specifically in ring theory, a Euclidean domain is an integral domain that can be endowed with a Euclidean function which allows a suitable generalization of the Euclidean division of integers. This generalized Euclidean algorithm can be put to many of the same uses as Euclid's original algorithm in the ring of integers: in any Euclidean domain, one can apply the Euclidean algorithm to compute the greatest common divisor of any two elements. In particular, the greatest common divisor of any two elements exists and can be written as a linear combination of them. Also every ideal in a Euclidean domain is principal, which implies a suitable generalization of the fundamental theorem of arithmetic: every Euclidean domain is a unique factorization domain.

In mathematics, particularly in algebra, a field extension is a pair of fields , such that the operations of K are those of L restricted to K. In this case, L is an extension field of K and K is a subfield of L. For example, under the usual notions of addition and multiplication, the complex numbers are an extension field of the real numbers; the real numbers are a subfield of the complex numbers.

In mathematics, an integral domain is a nonzero commutative ring in which the product of any two nonzero elements is nonzero. Integral domains are generalizations of the ring of integers and provide a natural setting for studying divisibility. In an integral domain, every nonzero element a has the cancellation property, that is, if a ≠ 0, an equality ab = ac implies b = c.

In mathematics, a polynomial is a mathematical expression consisting of indeterminates and coefficients, that involves only the operations of addition, subtraction, multiplication and exponentiation to nonnegative integer powers, and has a finite number of terms. An example of a polynomial of a single indeterminate x is x2 − 4x + 7. An example with three indeterminates is x3 + 2xyz2yz + 1.

In mathematics, a principal ideal domain, or PID, is an integral domain in which every ideal is principal. Some authors such as Bourbaki refer to PIDs as principal rings.

In mathematics, rings are algebraic structures that generalize fields: multiplication need not be commutative and multiplicative inverses need not exist. Informally, a ring is a set equipped with two binary operations satisfying properties analogous to those of addition and multiplication of integers. Ring elements may be numbers such as integers or complex numbers, but they may also be non-numerical objects such as polynomials, square matrices, functions, and power series.

In mathematics, a unique factorization domain (UFD) is a ring in which a statement analogous to the fundamental theorem of arithmetic holds. Specifically, a UFD is an integral domain in which every non-zero non-unit element can be written as a product of irreducible elements, uniquely up to order and units.

<span class="mw-page-title-main">Factorization</span> (Mathematical) decomposition into a product

In mathematics, factorization (or factorisation, see English spelling differences) or factoring consists of writing a number or another mathematical object as a product of several factors, usually smaller or simpler objects of the same kind. For example, 3 × 5 is an integer factorization of 15, and (x – 2)(x + 2) is a polynomial factorization of x2 – 4.

In algebraic number theory, an algebraic integer is a complex number that is integral over the integers. That is, an algebraic integer is a complex root of some monic polynomial whose coefficients are integers. The set of all algebraic integers A is closed under addition, subtraction and multiplication and therefore is a commutative subring of the complex numbers.

Hilbert's tenth problem is the tenth on the list of mathematical problems that the German mathematician David Hilbert posed in 1900. It is the challenge to provide a general algorithm that, for any given Diophantine equation, can decide whether the equation has a solution with all unknowns taking integer values.

In mathematics, an irreducible polynomial is, roughly speaking, a polynomial that cannot be factored into the product of two non-constant polynomials. The property of irreducibility depends on the nature of the coefficients that are accepted for the possible factors, that is, the ring to which the coefficients of the polynomial and its possible factors are supposed to belong. For example, the polynomial x2 − 2 is a polynomial with integer coefficients, but, as every integer is also a real number, it is also a polynomial with real coefficients. It is irreducible if it is considered as a polynomial with integer coefficients, but it factors as if it is considered as a polynomial with real coefficients. One says that the polynomial x2 − 2 is irreducible over the integers but not over the reals.

In ring theory, a branch of mathematics, the radical of an ideal of a commutative ring is another ideal defined by the property that an element is in the radical if and only if some power of is in . Taking the radical of an ideal is called radicalization. A radical ideal is an ideal that is equal to its radical. The radical of a primary ideal is a prime ideal.

In algebra, a monic polynomial is a non-zero univariate polynomial in which the leading coefficient is equal to 1. That is to say, a monic polynomial is one that can be written as

In mathematics, the characteristic of a ring R, often denoted char(R), is defined to be the smallest positive number of copies of the ring's multiplicative identity (1) that will sum to the additive identity (0). If no such number exists, the ring is said to have characteristic zero.

In commutative algebra and field theory, the Frobenius endomorphism is a special endomorphism of commutative rings with prime characteristic p, an important class that includes finite fields. The endomorphism maps every element to its p-th power. In certain contexts it is an automorphism, but this is not true in general.

In ring theory, a branch of mathematics, a ring R is a polynomial identity ring if there is, for some N > 0, an element P ≠ 0 of the free algebra, ZX1, X2, ..., XN, over the ring of integers in N variables X1, X2, ..., XN such that

In mathematics, an algebraic number field is an extension field of the field of rational numbers such that the field extension has finite degree . Thus is a field that contains and has finite dimension when considered as a vector space over .

References

  1. Julio R. Bastida, Field Extensions and Galois Theory, Addison-Wesley Publishing Company, 1984, p.8.
  2. Fraleigh, John B., A First Course in Abstract Algebra, Addison-Wesley Publishing Company, 1993, p.453, ISBN   0-201-53467-3.
  3. Difusión DM (2018-02-23), Introduction to Tropical Algebraic Geometry (1 of 5) , retrieved 2019-06-11
  4. 1 2 A. Granville, It Is Easy To Determine Whether A Given Integer Is Prime , Bull. of the AMS, Volume 42, Number 1 (Sep. 2004), Pages 3–38.
  5. Original source: Gleason, Harold Willard (September 6, 1938), ""Dark and Bloody Ground---" (The Freshman's Dream)", The New York Sun. Reproduced in:
  6. Colin R. Fletcher, Review of Selected papers on algebra, edited by Susan Montgomery, Elizabeth W. Ralston and others. Pp xv, 537. 1977. ISBN   0-88385-203-9 (Mathematical Association of America), The Mathematical Gazette, Vol. 62, No. 421 (Oct., 1978), The Mathematical Association. p. 221.
  7. Thomas W. Hungerford, Algebra, Springer, 1974, p. 121 (with McBrien's name also stated on pp. ix and 498); also in Abstract Algebra: An Introduction, 2nd edition. Brooks Cole, July 12, 1996, p. 366.
  8. John B. Fraleigh, A First Course In Abstract Algebra, 6th edition, Addison-Wesley, 1998. pp. 262 and 438.
  9. Google books 1800–1900 search for "freshman's dream": Bentley's miscellany, Volume 26, p. 176, 1849