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 A073725 #9 Jan 03 2017 02:31:06 %S A073725 0,0,1,2,1,1,2,3,3,2,3,6,5,3,11,6,10,6,3,9,11,9,14,6,10,14,9,6,11,14, %T A073725 13,9,28,11,20,14,10,27,14,24,18,9,19,24,20,32,11,20,29,14,14,24,27, %U A073725 24,42,14,20,37,27,14,45,28,39,27,14,51,29,42,31,51,20,28,42,27,20,32,32 %N A073725 a(n)-th composite number = phi(n-th composite number); a(1)=a(2)=0. %F A073725 See program. %e A073725 Phi of 25th composite number = 10th composite number: n=25: A002808(25)=38, phi(38) = 18 = A002808(10) so a(25)=10. %t A073725 c[x_] := FixedPoint[x+PrimePi[ # ]+1&, x]; G[x_] := x-PrimePi[x]-1; a(n)=G[EulerPhi[c[n]]] %Y A073725 Cf. A000010, A002808, A062298. %K A073725 nonn %O A073725 1,4 %A A073725 _Labos Elemer_, Aug 05 2002