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 A121291 #4 Mar 30 2012 18:40:38 %S A121291 1,5,12,21,31,43,57,73,92,112,135,160,187,215,246,278,318,353,392,432, %T A121291 477,525,576,628,683,739,798,857,920,986,1055,1128,1203 %N A121291 Rounded partial sum of atomic weights of first n elements. %e A121291 a(1) = round(1.0080) = 1 = mean atomic weight of hydrogen (protium + deuterium + tritium). %e A121291 a(2) = round (1.0080+4.003) = round(5.011) = 5 (heium 4 and rarer helium 3). %Y A121291 Cf. A007656, A070217. %K A121291 nonn %O A121291 1,2 %A A121291 _Jonathan Vos Post_, Aug 24 2006