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 A020950 #10 May 05 2023 12:29:47 %S A020950 0,2,4,8,10,32,18,20,34,38,36,44,42,68,72,92,76,74,82,188,84,140,138, %T A020950 152,150,146,154,266,148,164,172,278,274,170,282,314,276,536,324,296, %U A020950 292,578,300,308,364,332,298,566,330,338,552,548,562,1274,340,584,564,614,628 %N A020950 a(n) = k-1, where k is smallest number such that A002487(k) = n. %e A020950 A002487(33) = 6 and this is the first time 6 appears, so a(6) = 33-1 = 32. %o A020950 (Python) %o A020950 from itertools import count %o A020950 from functools import reduce %o A020950 def A020950(n): return next(filter(lambda k:sum(reduce(lambda x,y:(x[0],x[0]+x[1]) if int(y) else (x[0]+x[1],x[1]),bin(k)[-1:2:-1],(1,0)))==n,count(1)))-1 # _Chai Wah Wu_, May 05 2023 %Y A020950 Equals A020946 - 1. %Y A020950 Cf. A020943-A020946, A002487, A020947-A020949. %K A020950 nonn %O A020950 1,2 %A A020950 _Clark Kimberling_ %E A020950 Corrected and extended by _David W. Wilson_