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 A305823 #7 Jun 10 2018 13:33:07
%S A305823 0,1,3,8,20,53,136,383,1067,3292,10162,34586,118332,441898,1660947,
%T A305823 6745710,27575425,120801230,532530170,2499230441,11800052796,
%U A305823 58992236094,296630364605,1572128197439,8378539879636,46882666240016,263735138346443,1552510947814406
%N A305823 Total number of blocks in all set partitions of [n] with alternating parity of elements.
%H A305823 Wikipedia, <a href="https://en.wikipedia.org/wiki/Partition_of_a_set">Partition of a set</a>
%F A305823 a(n) = Sum_{k=0..n} k * A274581(n,k).
%Y A305823 Cf. A274581.
%K A305823 nonn
%O A305823 0,3
%A A305823 _Alois P. Heinz_, Jun 10 2018