123132231213312321what is the name of combinatorial mathematics?

That is, Ann (where one n is in the upper right corner of a and one is in the lower right corner! )

The algorithm is

If A44 is 4×3×2× 1.

A55 is 5×4×3×2× 1.

According to equivalent reasoning

If A26 (upper 2, lower 6) is equal to 6×5.

A37 is 7×6×5.

The number above is the number of multiplied numbers, gradually decreasing, and the number below is the first number at the beginning!