Characters["string"] gives a list of the characters in a string.

Element[x, dom] or x \[Element] dom asserts that x is an element of the domain dom. Element[{x_1, x_2, ...}, dom] asserts that all the x_i are elements of dom. Element[patt, ...

ImageDeconvolve[image, ker] gives a deconvolution of image using kernel ker.

Integers represents the domain of integers, as in x \[Element] Integers.

Reap[expr] gives the value of expr together with all expressions to which Sow has been applied during its evaluation. Expressions sown using Sow[e] or Sow[e, tag_i] with ...

SequenceAlignment[s_1, s_2] finds an optimal alignment of sequences of elements in the strings or lists s_1 and s_2, and yields a list of successive matching and differing ...

StringCount["string", " sub"] gives a count of the number of times " sub" appears as a substring of " string". StringCount["string", patt] gives the number of substrings in " ...

Tally[list] tallies the elements in list, listing all distinct elements together with their multiplicities.Tally[list, test] uses test to determine whether pairs of elements ...

Finding the structure of polynomials written in expanded form. Here is a polynomial in two variables. This is the polynomial in expanded form.

Functions like Factor usually assume that all coefficients in the polynomials they produce must involve only rational numbers. But by setting the option Extension you can ...