**IFS**
An iterated functional system; it constructs a fractal by
iterating a vector quantity through an affine equation that
is randomly selected on each iteration.

**Imaginary Number**
The square root of a negative number. The square root of -1 is often
denoted as *i* for the purpose of writing out complex numbers.

**Implicit Parallelism**
The idea that genetic algorithms have an extra built-in form of
parallelism that is expressed when a GA searches through a
search space. Implicit parallelism depends on the similarities
and differences between individuals in the population. The theory
posits that GAs process more schemata than there are strings
in a population, thus getting something of a free lunch. See also
no free lunch.

**Incomputable**
Something that cannot be characterized by a program that always
halts. Sets that are incomputable may be recursively
enumerable (like the halting set),
co-recursively enumerable
(e.g., the halting set's complement),
or not recursively enumerable
(which, if also not CO-RE, is a random
set).

**Incomputable Number**
A real number with an infinite decimal (or binary) expansion
that cannot be enumerated by any universal computer.

**Inheritable**
Refers to a trait that can be genetically passed from parent to
offspring.

**Inhibitory**
Refers to a neural synapse or weight that is negative such
that activity in the source neuron encourages inactivity in the
connected neuron. The opposite of excitatory.

**Inner Product**
For two vectors of the same dimensionality, the sum of the
pairwise products of the two vector components.

**Integral**
The cumulative continuous sum of a function. The integral
of a differential equation represents the future state of a
dynamical system; however, most integrals do not have an
analytical solution, which means that they may only have
numerical solutions, an admittedly inexact process.

**Integration**
The act of calculating an integral, by either a numerical or
an analytical solution; the inverse operation of
differentiation.

**Invertible**
A function is invertible (with a unique inverse) if the output
uniquely determines the input (i.e., it is one-to-one) and the
set of legal outputs is equal to the set of legal inputs. The
function *x^2* is not strictly invertible, while *x^3* has an inverse.
For operations the definition is slightly looser. While
integration and differentiation are considered to be
inverse operations, there are an infinite number of
integrals that are valid results for integrating any function;
thus, the process is not one-to-one.

**Irrational Number**
A real number that cannot be represented as a fraction.

**Iterated Prisoner's Dilemma**
The Prisoner's Dilemma game played in an iterative manner
for a number of rounds that is unknown to both players.

**Iterate/Iterative**
Doing something repeatedly. Doing something repeatedly. Doing
something repeatedly. Doing something repeatedly. Doing something
repeatedly.