crossprod() function returns matrix cross-product.

crossprod(x, y = NULL)
tcrossprod(x, y = NULL)

x: numeric matrix
y: numeric matrix, if y=NULL, y is the same as x
...

> x <- matrix(1:9,3,3)
> x

[,1] [,2] [,3]
[1,] 1 4 7
[2,] 2 5 8
[3,] 3 6 9

> crossprod(x)

[,1] [,2] [,3]
[1,] 14 32 50
[2,] 32 77 122
[3,] 50 122 194

> tcrossprod(x)

[,1] [,2] [,3]
[1,] 66 78 90
[2,] 78 93 108
[3,] 90 108 126