Basic Linear Algebra Subroutines (BLAS) are routines that provide standard functions for basic vector and matrix operations. You can classify these routines in the following three levels:

• BLAS level 1, which calculates vector operations.
• BLAS level 2, which calculates matrix-vector operations.
• BLAS level 3, which calculates matrix-matrix operations.

For example, dgemm is a BLAS level 3 function that calculates the general real matrix-matrix product using the following expression:

where α and β are scalars, A, B, and C are matrices, and opA is A or A^T.

Refer to BLAS (Basic Linear Algebra Subprograms) for more information on the BLAS functions.