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 A210500 #18 Jan 21 2013 15:02:26
%S A210500 1,2,3,2,1,4,5,2,1,1,1,5,6,2,1,1,1,5,1,1,11,1,1,4,13,2,1,1,5,4,1,1,1,
%T A210500 1,1,1,4,2,1,1,1,5,1,17,1,1,1,1,1,1,4,1,21,1,9,1,1,1,5,5,1,1,1,1,10,1,
%U A210500 1,13,1,3,9,1,1,1,1,1,1,7,9,4,1,7,1,23,1,1,9
%N A210500 Number of even solutions to phi(k) = prime(n) - 1.
%C A210500 a(n) >= A210501(n).
%D A210500 Alexander S. Karpenko, Lukasiewicz's Logics and Prime Numbers, Luniver Press, Beckington, 2006, pp. 52-56.
%H A210500 Arkadiusz Wesolowski, <a href="/A210500/b210500.txt">Table of n, a(n) for n = 1..1000</a>
%F A210500 a(n) = A058339(n) - A210501(n).
%e A210500 The set {k: phi(k) = 12} is {13, 21, 26, 28, 36, 42}. Thus, if phi(k) = prime(6) - 1, the equation has exactly four even solutions. Hence, a(6) = 4.
%t A210500 r = 87; lst1 = Table[EulerPhi[n], {n, (Prime[r] - 1)^2 + 2}]; lst2 = {}; Do[p = Prime[n]; AppendTo[lst2, Length@Select[Flatten@Position[Take[lst1, {p - 1, (p - 1)^2 + 2}], Prime[n] - 1], OddQ]], {n, r}]; lst2
%Y A210500 Cf. A000010, A032446, A058339, A066080, A210501, A210502.
%K A210500 nonn
%O A210500 1,2
%A A210500 _Arkadiusz Wesolowski_, Jan 19 2013