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 A121818 #27 Mar 16 2024 04:25:06
%S A121818 1,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,20,21,22,23,24,25,26,27,
%T A121818 28,30,31,32,33,34,35,36,38,39,40,41,43,44,45,46,47,49,50,51,52,53,55,
%U A121818 56,57,58,60,61,62,63,65,66,67,69,70,71,72,74,75,76,78,79,80,81,83,84,85
%N A121818 Average of atomic weights of all isotopes of first n elements, rounded to nearest integer.
%C A121818 This is poorly defined, even with the caveats of A007656, for the highest atomic numbers (with no stable isotopes) and the values given are with respect to Earth, as different planets presumably have slightly different isotopic ratios.
%e A121818 a(1) = round(1.0080/1) = 1 = mean atomic weight of hydrogen (protium + deuterium + tritium).
%e A121818 a(2) = round((1.0080+4.003)/2) = round(5.011/2) = round(2.5055) = 3.
%t A121818 Needs["Miscellaneous`ChemicalElements`"]
%t A121818 Table[ Round[ Sum[ AtomicWeight(AT)Elements[[i]], {i, n}]/n], {n, 111}] (* _Robert G. Wilson v_, Sep 02 2006 *)
%Y A121818 Cf. A007656, A070217, A121291.
%K A121818 nonn,fini,less
%O A121818 1,2
%A A121818 _Jonathan Vos Post_, Aug 26 2006
%E A121818 More terms from _Robert G. Wilson v_, Sep 02 2006
%E A121818 Edited by _N. J. A. Sloane_, Sep 14 2006