Combinatorics: nPr, nCr, n!

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Count ordered selections ($P(n,k)$), unordered selections ($C(n,k)$), or compute $n!$. Messages warn when IEEE doubles cannot represent the exact integer.

Quantity n k (ignored for n!)

What it computes. Standard counting formulas with multiplicative loops / log-space fallbacks.

Limitations. $n!$ overflows past 170 in double precision; huge $C(n,k)$ may only be approximate. See the disclaimer.