Distinct terms in a multiplication table

Let P(m,n) be the number of distinct terms in an m×n multiplication table.

For example, a 3×4 multiplication table looks like this:

There are 8 distinct terms {1,2,3,4,6,8,9,12}, therefore P(3,4) = 8.

You are given that: P(64,64) = 1263, P(12,345) = 1998, and P(32,10¹⁵) = 13826382602124302.

Find P(64,10¹⁶).

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