This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A087181 #16 May 22 2025 09:54:14
%S A087181 1,2,2,2,2,3,3,3,4,4,4,4,4,4,4,4,4,4,5,6,6,6,6,7,7,7,7,8,9,9,9,9,9,9,
%T A087181 10,10,11,12,12,12,12,12,12,12,13,14,14,14,14,14,15,15,15,15,16,17,17,
%U A087181 17,17,18,18,18,19,19,20,20,20,20,20,20,20,20,21,22,22,22,22,23,23,23
%N A087181 Number partition numbers <= P(n) of the form 3*k+1 (P = A000041).
%H A087181 Amiram Eldar, <a href="/A087181/b087181.txt">Table of n, a(n) for n = 0..10000</a>
%H A087181 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/PartitionFunction.html">Partition Function</a>.
%H A087181 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/PartitionFunctionPCongruences.html">Partition Function P Congruences</a>.
%F A087181 A087180(n) + a(n) + A087182(n) = n + 1.
%t A087181 Accumulate[Table[Boole[Mod[PartitionsP[n], 3] == 1], {n, 0, 100}]] (* _Amiram Eldar_, May 22 2025 *)
%Y A087181 Cf. A000041, A068907, A071754, A087177, A087180, A087182, A087184.
%K A087181 nonn
%O A087181 0,2
%A A087181 _Reinhard Zumkeller_, Aug 23 2003