Euler’s pentagonal number theorem (1750):

http://mathworld.wolfram.com/PartitionFunctionP.html, equation (11).

   pn  ← {1≥⍵:0≤⍵ ⋄ -/+⌿∇¨rec ⍵}
   rec ← {⍵ - (÷∘2 (×⍤1) ¯1 1 ∘.+ 3∘×) 1+⍳⌈0.5*⍨⍵×2÷3}

   pn¨ 0 1 2 3 4 5 6
1 1 2 3 5 7 11

   pn 30
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