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 A202534 #14 Feb 26 2015 09:55:51 %S A202534 0,0,3,49,972,32565,2096275,268431316,68719455589,35184371972857, %T A202534 36028797018285398,73786976294833992867,302231454903657266032107, %U A202534 2475880078570760549607349126,40564819207303340847893119613487,1329227995784915872903807049800202429 %N A202534 Number of symmetric, reflexive, non-transitive relations on n elements. %C A202534 Of the values shown, only 3 is prime. Are there any other prime values in the sequence? - _Jonathan Vos Post_, Dec 29 2011 %F A202534 a(n) = 2^(n*(n-1)/2) - A000110(n) = A006125(n) - A000110(n). %e A202534 The first symmetric, reflexive, nontransitive relation occurs for n=3: omitting a non-identical couple (a,b) from the total relation gives such a relation (and for n=3, this is the only way). There are 3 ways to choose this couple. %o A202534 (Sage) def a(n): return 2^(n*(n-1)/2) - bell_number(n) %K A202534 nonn %O A202534 1,3 %A A202534 _Bert Seghers_, Dec 20 2011