Partitions into a small number of part sizes

We study $ν_k(n)$, the number of partitions of $n$ into $k$ part sizes, and find numerous arithmetic progressions where $ν_2$ and $ν_3$ take on values divisible by 2 and 4. Expanding earlier work, we show $ν_2(An+B) \equiv 0 \pmod{4}$ for (A,B) = (36,30), (72,42), (252,114), (196,70), and likely many other progressions for which our method should easily generalize. Of some independent interest, we prove that the overpartition function $\bar{p}(n) \equiv 0 \pmod{16}$ in the first three progressions (the fourth is known), and thereby show that $ν_3(An+B) \equiv 0 \pmod{2}$ in each of these progressions as well, and discuss the relationship between these congruences in more generality. We end with open questions in this area.

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