In mathematics, an **index set** is a set whose members label (or index) members of another set.^{ [1] }^{ [2] } For instance, if the elements of a set A may be *indexed* or *labeled* by means of the elements of a set J, then J is an index set. The indexing consists of a surjective function from J onto A, and the indexed collection is typically called an * (indexed) family *, often written as {*A*_{j}}_{j∈J}.

- An enumeration of a set S gives an index set , where
*f*:*J*→*S*is the particular enumeration of*S*. - Any countably infinite set can be (injectively) indexed by the set of natural numbers .
- For , the indicator function on
*r*is the function given by

The set of all such indicator functions, , is an uncountable set indexed by .

In computational complexity theory and cryptography, an index set is a set for which there exists an algorithm I that can sample the set efficiently; e.g., on input 1* ^{n}*, I can efficiently select a poly(n)-bit long element from the set.

In mathematics, one can often define a **direct product** of objects already known, giving a new one. This generalizes the Cartesian product of the underlying sets, together with a suitably defined structure on the product set. More abstractly, one talks about the product in category theory, which formalizes these notions.

In mathematics, a **product** is the result of multiplication, or an expression that identifies factors to be multiplied. For example, 30 is the product of 6 and 5, and is the product of and .

In computability theory, **Rice's theorem** states that all non-trivial, semantic properties of programs are undecidable. A semantic property is one about the program's behavior, unlike a syntactic property. A property is *non-trivial* if it is neither true for every computable function, nor false for every computable function.

In mathematical analysis, **semi-continuity** is a property of extended real-valued functions that is weaker than continuity. An extended real-valued function *f* is **upper****semi-continuous** at a point *x*_{0} if, roughly speaking, the function values for arguments near *x*_{0} are not much higher than *f*(*x*_{0}).

An **enumeration** is a complete, ordered listing of all the items in a collection. The term is commonly used in mathematics and computer science to refer to a listing of all of the elements of a set. The precise requirements for an enumeration depend on the discipline of study and the context of a given problem.

In mathematics, an **indicator function** or a **characteristic function** is a function defined on a set X that indicates membership of an element in a subset A of X, having the value 1 for all elements of A and the value 0 for all elements of X not in A. It is usually denoted by a symbol 1 or I, sometimes in boldface or blackboard boldface, with a subscript specifying the subset.

In mathematics, a **multiset** is a modification of the concept of a set that, unlike a set, allows for multiple instances for each of its elements. The positive integer number of instances, given for each element is called the **multiplicity** of this element in the multiset. As a consequence, an infinite number of multisets exist, which contain only elements *a* and *b*, but vary by the multiplicity of their elements:

In combinatorics, a branch of mathematics, the **inclusion–exclusion principle** is a counting technique which generalizes the familiar method of obtaining the number of elements in the union of two finite sets; symbolically expressed as

In mathematics, the **Hessian matrix** or **Hessian** is a square matrix of second-order partial derivatives of a scalar-valued function, or scalar field. It describes the local curvature of a function of many variables. The Hessian matrix was developed in the 19th century by the German mathematician Ludwig Otto Hesse and later named after him. Hesse originally used the term "functional determinants".

In mathematics, a **spline** is a special function defined piecewise by polynomials. In interpolating problems, spline interpolation is often preferred to polynomial interpolation because it yields similar results, even when using low degree polynomials, while avoiding Runge's phenomenon for higher degrees.

In geometry and group theory, a **lattice** in is a subgroup of the additive group which is isomorphic to the additive group , and which spans the real vector space . In other words, for any basis of , the subgroup of all linear combinations with integer coefficients of the basis vectors forms a lattice. A lattice may be viewed as a regular tiling of a space by a primitive cell.

**Hyperelliptic curve cryptography** is similar to elliptic curve cryptography (ECC) insofar as the Jacobian of a hyperelliptic curve is an abelian group in which to do arithmetic, just as we use the group of points on an elliptic curve in ECC.

In the mathematical field of graph theory, the **Laplacian matrix**, also called the **graph Laplacian**, **admittance matrix**, **Kirchhoff matrix** or **discrete Laplacian**, is a matrix representation of a graph. The Laplacian matrix can be used to find many useful properties of a graph. Together with Kirchhoff's theorem, it can be used to calculate the number of spanning trees for a given graph. The sparsest cut of a graph can be approximated through the second smallest eigenvalue of its Laplacian by Cheeger's inequality. It can also be used to construct low dimensional embeddings, which can be useful for a variety of machine learning applications.

In set theory, the **cardinality of the continuum** is the cardinality or "size" of the set of real numbers , sometimes called the continuum. It is an infinite cardinal number and is denoted by or .

In mathematics, an **iterated binary operation** is an extension of a binary operation on a set *S* to a function on finite sequences of elements of *S* through repeated application. Common examples include the extension of the addition operation to the summation operation, and the extension of the multiplication operation to the product operation. Other operations, e.g., the set theoretic operations union and intersection, are also often iterated, but the iterations are not given separate names. In print, summation and product are represented by special symbols; but other iterated operators often are denoted by larger variants of the symbol for the ordinary binary operator. Thus, the iterations of the four operations mentioned above are denoted

In the field of mathematical modeling, a **radial basis function network** is an artificial neural network that uses radial basis functions as activation functions. The output of the network is a linear combination of radial basis functions of the inputs and neuron parameters. Radial basis function networks have many uses, including function approximation, time series prediction, classification, and system control. They were first formulated in a 1988 paper by Broomhead and Lowe, both researchers at the Royal Signals and Radar Establishment.

In probability theory and statistics, a **categorical distribution** is a discrete probability distribution that describes the possible results of a random variable that can take on one of *K* possible categories, with the probability of each category separately specified. There is no innate underlying ordering of these outcomes, but numerical labels are often attached for convenience in describing the distribution,. The *K*-dimensional categorical distribution is the most general distribution over a *K*-way event; any other discrete distribution over a size-*K* sample space is a special case. The parameters specifying the probabilities of each possible outcome are constrained only by the fact that each must be in the range 0 to 1, and all must sum to 1.

**Learning with errors** (**LWE**) is the computational problem of inferring a linear -ary function over a finite ring from given samples some of which may be erroneous. The LWE problem is conjectured to be hard to solve, and thus be useful in cryptography.

In 1997, Moni Naor and Omer Reingold described efficient constructions for various cryptographic primitives in private key as well as public-key cryptography. Their result is the construction of an efficient pseudorandom function. Let *p* and *l* be prime numbers with *l* |*p*−1. Select an element *g* ∈ of multiplicative order *l*. Then for each *n*-dimensional vector *a* = ∈ they define the function

In discrete mathematics, **ideal lattices** are a special class of lattices and a generalization of cyclic lattices. Ideal lattices naturally occur in many parts of number theory, but also in other areas. In particular, they have a significant place in cryptography. Micciancio defined a generalization of cyclic lattices as ideal lattices. They can be used in cryptosystems to decrease by a square root the number of parameters necessary to describe a lattice, making them more efficient. Ideal lattices are a new concept, but similar lattice classes have been used for a long time. For example cyclic lattices, a special case of ideal lattices, are used in NTRUEncrypt and NTRUSign.

- ↑ Weisstein, Eric. "Index Set".
*Wolfram MathWorld*. Wolfram Research. Retrieved 30 December 2013. - ↑ Munkres, James R. (2000).
*Topology*.**2**. Upper Saddle River: Prentice Hall. - ↑ Goldreich, Oded (2001).
*Foundations of Cryptography: Volume 1, Basic Tools*. Cambridge University Press. ISBN 0-521-79172-3.

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