# Sum of squares

In mathematics, statistics and elsewhere, sums of squares occur in a number of contexts:

## Algebra and algebraic geometry

• For representing a polynomial as the sum of squares of polynomials, see Polynomial SOS.
• For representing a multivariate polynomial that takes only non-negative values over the reals as a sum of squares of rational functions, see Hilbert's seventeenth problem.
• The Brahmagupta–Fibonacci identity says the set of all sums of two squares is closed under multiplication.
• The sum of squared dimensions of a finite group's pairwise nonequivalent complex representations is equal to cardinality of that group.

## Euclidean geometry and other inner-product spaces

• The Pythagorean theorem says that the square on the hypotenuse of a right triangle is equal in area to the sum of the squares on the legs. The sum of squares is not factorable.
• The Squared Euclidean distance (SED) is defined as the sum of squares of the differences between coordinates.
• Heron's formula for the area of a triangle can be re-written as using the sums of squares of a triangle's sides (and the sums of the squares of squares)
• The British flag theorem for rectangles equates two sums of two squares
• The parallelogram law equates the sum of the squares of the four sides to the sum of the squares of the diagonals
• Descartes' theorem for four kissing circles involves sums of squares
• The sum of the squares of the edges of a rectangular cuboid equals the square of any space diagonal