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 A305739 #16 Jun 25 2018 09:33:43 %S A305739 0,5,35,239,1799,15119,141119,1451519,16329599,199583999,2634508799, %T A305739 37362124799,566658892799,9153720575999,156920924159999, %U A305739 2845499424767999,54420176498687999,1094805903679487999,23112569077678079999,510909421717094399999 %N A305739 a(n) = n!*T(n) - 1, where T(n) is the n-th triangular number. %F A305739 a(n) = n*(n+1)!/2 - 1. - _Michel Marcus_, Jun 23 2018 %F A305739 a(n) = A180119(n)-1 = A001286(n+1)-1. - _Alois P. Heinz_, Jun 24 2018 %p A305739 seq(n*(n+1)!/2-1,n=1..21); %o A305739 (PARI) a(n) = n*(n+1)!/2 - 1; \\ _Michel Marcus_, Jun 23 2018 %Y A305739 Cf. A000142, A000217, A001286, A180119. %Y A305739 See A305738 for the indices of primes in this sequence. %K A305739 nonn %O A305739 1,2 %A A305739 _Maheswara Rao Valluri_, Jun 22 2018