Diagonal[m] gives the list of elements on the leading diagonal of the matrix m.Diagonal[m, k] gives the elements on the k\[Null]^th diagonal of m.

FixedPointList[f, expr] generates a list giving the results of applying f repeatedly, starting with expr, until the results no longer change.

MovingAverage[list, r] gives the moving average of list, computed by averaging runs of r elements.MovingAverage[list, {w_1, w_2, ..., w_r}] gives the moving average of list, ...

Covariance[v_1, v_2] gives the covariance between the vectors v_1 and v_2.Covariance[m] gives the covariance matrix for the matrix m.Covariance[m_1, m_2] gives the covariance ...

Length[expr] gives the number of elements in expr.

Quaternion[a, b, c, d] represents the quaternion a + I b + J c + K d.

ComposeList[{f_1, f_2, ...}, x] generates a list of the form {x, f_1[x], f_2[f_1[x]], ...}.

Disjunction[expr, {a_1, a_2, ...}] gives the disjunction of expr over all choices of the Boolean variables a_i.

NullRecords is an option for Read and related functions which specifies whether null records should be taken to exist between repeated record separators.

Built into Mathematica are a large number of special characters intended for use in mathematical and other notation. "Listing of Named Characters" gives a complete listing. ...