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 A275781 #11 Aug 20 2024 14:14:12 %S A275781 0,0,0,1,3,9,43,209,1042,5682,32875,200919,1299092,8848623,63108778, %T A275781 471509591,3671980053,29797471739,251120234182,2196428139810, %U A275781 19882638695048,186205573812059,1799766634300161,17946783669353462,184277407872729741,1947501874827169041 %N A275781 Number of set partitions of [n] having at least two blocks with equal element sums. %F A275781 a(n) = A000110(n) - A275780(n). %e A275781 a(3) = 1: 12|3 %e A275781 a(4) = 3: 12|3|4, 13|2|4, 14|23. %e A275781 a(5) = 9: 12|3|45, 12|3|4|5, 13|25|4, 13|2|4|5, 14|23|5, 1|23|4|5, 14|2|3|5, 15|24|3, 1|25|34. %Y A275781 Cf. A000110, A035470, A275780. %K A275781 nonn %O A275781 0,5 %A A275781 _Alois P. Heinz_, Aug 08 2016 %E A275781 a(17)-a(25) from _Christian Sievers_, Aug 20 2024