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 A349723 #84 Jan 01 2022 00:21:28 %S A349723 1,6,14,27,45,70,102,143,193,254,326,411,509,622,750,895,1057,1238, %T A349723 1438,1659,1901,2166,2454,2767,3105,3470,3862,4283,4733,5214,5726, %U A349723 6271,6849,7462,8110,8795,9517,10278,11078,11919,12801,13726,14694,15707 %N A349723 Atomic number corresponding to the element that is the first of the two middle elements in the n-th row of the periodic table of elements. %C A349723 Elements corresponding to the initial sequence terms are hydrogen, carbon, silicon, cobalt, rhodium, ytterbium, and nobelium. However, there may not be a physical realization for higher terms. %H A349723 <a href="/index/Rec#order_05">Index entries for linear recurrences with constant coefficients</a>, signature (3,-2,-2,3,-1). %F A349723 a(n) = A018227(n-1) + A093907(n)/2 for n > 1. %F A349723 a(n) = (A018227(n) + A018227(n-1))/2 for n > 1. %F A349723 a(n) = (4*n^3 + 18*n^2 + 32*n + 3*(-1)^n - 27)/24. %F A349723 G.f.: x*(1 + 3*x - 2*x^2 - x^3 + x^4)/((1 - x)^4*(1 + x)). - _Stefano Spezia_, Dec 31 2021 %e A349723 For n=1, hydrogen (with atomic number 1) is the first element of two middle elements in the 1st row of periodic table. %e A349723 For n=2, carbon (with atomic number 6) is the first element of the two middle elements in the 2nd row of periodic table. %e A349723 In general, the n-th number is the addition of the atomic number of the first lower noble element plus half the number of elements in the current row of the periodic table. %Y A349723 Cf. A018227, A093907. %K A349723 nonn,easy %O A349723 1,2 %A A349723 _Yigit Oktar_, Dec 31 2021