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 A328823 #33 Nov 13 2019 11:06:02 %S A328823 2,5,3,2,2,3,5,2,2,5,7,2,2,7,3,2,2,3,11,2,2,5,13,2,2,13,3,2,2,3,17,2, %T A328823 2,17,5,2,2,19,3,2,2,3,23,2,2,7,5,2,2,5,3,2,2,3,5,2,2,29,31,2,2,5,3,2, %U A328823 2,3,5,2,2,5,37,2,2,7,3,2,2 %N A328823 a(n) is the least prime factor of A000096(n) = n*(n+3)/2. %C A328823 The motivation is that n*(n+3)/2 = n + T(n), where T(n) = n*(n+1)/2 = A000217(n), the triangular numbers. %F A328823 a(n) = A020639(A000096(n)). - _Michel Marcus_, Oct 28 2019 %e A328823 For n = 5, n + T(n) = 5 + 15 = 20. The smallest prime factor of 20 is 2, so a(5) = 2. %t A328823 Array[FactorInteger[# (# + 3)/2][[1, 1]] &, 77] (* _Amiram Eldar_, Nov 02 2019 *) %o A328823 (PARI) A328823(n)=A020639(n*(n+3)/2) \\ _M. F. Hasler_, Nov 10 2019 %Y A328823 Cf. A000096 (n + T(n)), A000217 (triangular numbers), A020639 (smallest prime factor). %K A328823 nonn %O A328823 1,1 %A A328823 _Ali Sada_, Oct 28 2019 %E A328823 Simpler definition from _M. F. Hasler_, Nov 10 2019