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 A212849 #14 May 02 2018 09:27:58
%S A212849 1,3,8,9,21,25,26,30,32,36,38,44,68,86,105,106,112,115,125,126,138,
%T A212849 150,155,160,164,178,180,186,187,192,195,203,206,208,216,231,234,243,
%U A212849 266,275,290,299,302,305,323,330,338,343,348,352,365,366,380,396,404,413
%N A212849 Numbers whose sum of prime factors (counted with multiplicity) is a triangular number.
%C A212849 This is to A212831 Numbers whose sum of prime factors is a square (counted with multiplicity) as A000290 squares are to A000217 triangular numbers.
%H A212849 G. C. Greubel, <a href="/A212849/b212849.txt">Table of n, a(n) for n = 1..5000</a>
%F A212849 {k such that A001414(k) = sopfr(k) is in A000217}.
%e A212849 sopfr(21) = sum of primes dividing 21 (with repetition) = 10, which is the 4th triangular number, so 21 is in this sequence.
%e A212849 The number 1 is here because the sum of its prime factors is 0, which is a triangular number.
%t A212849 triangularQ[n_] := IntegerQ[Sqrt[1 + 8*n]]; fQ[n_] := Module[{p, e}, {p, e} = Transpose[FactorInteger[n]]; triangularQ[Dot[p, e]]]; Join[{1}, Select[Range[2, 500], fQ]] (* _T. D. Noe_, May 30 2012 *)
%o A212849 (PARI) sopfr(n) = my(f=factor(n)); sum(k=1, matsize(f)[1], f[k, 1]*f[k, 2]); \\ A001414
%o A212849 isok(n) = ispolygonal(sopfr(n), 3); \\ _Michel Marcus_, May 02 2018
%Y A212849 Cf. A000217, A001414, A212831.
%K A212849 nonn,easy
%O A212849 1,2
%A A212849 _Jonathan Vos Post_, May 28 2012