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 A292874 #10 Sep 26 2017 02:34:24 %S A292874 2,3,5,7,4,13,10,19,8,29,21,37,20,43,22,34,18,61,67,9,73,57,44,40,52, %T A292874 101,63,85,109,74,93,86,137,76,149,91,157,163,32,173,88,117,68,193, %U A292874 197,199,211,84,80,229,36,104,241,96,257,102,136 %N A292874 Least k such that A039654(k) = prime(n). %C A292874 We have A039653(k) >= k, thus also A039654(k) >= k, with equality if k is prime, therefore a(n) <= prime(n), and the largest k for which A039654(k) = prime(n) is always k = prime(n). %o A292874 (PARI) a039654(n) = if(n<2, 0, my(m); while((m=sigma(n)-1)!=n, n=m); n); %o A292874 a(n) = my(k=0, p=prime(n)); while(a039654(k) != p, k++); k; \\ _Michel Marcus_, Sep 26 2017 %Y A292874 Cf. A039653, A039654, A177343, A292112, A292113. %K A292874 nonn %O A292874 1,1 %A A292874 _M. F. Hasler_, Sep 25 2017