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 A210501 #20 Jan 21 2013 15:03:00
%S A210501 1,1,1,2,1,2,1,2,1,1,1,3,3,2,1,1,1,4,1,1,6,1,1,2,4,2,1,1,4,2,1,1,1,1,
%T A210501 1,1,3,2,1,1,1,5,1,4,1,1,1,1,1,1,2,1,10,1,1,1,1,1,4,3,1,1,1,1,6,1,1,5,
%U A210501 1,3,3,1,1,1,1,1,1,6,4,2,1,6,1,11,1,1,3
%N A210501 Number of odd solutions to phi(k) = prime(n) - 1.
%C A210501 a(n) <= A210500(n).
%D A210501 Alexander S. Karpenko, Lukasiewicz's Logics and Prime Numbers, Luniver Press, Beckington, 2006, pp. 52-56.
%H A210501 Arkadiusz Wesolowski, <a href="/A210501/b210501.txt">Table of n, a(n) for n = 1..1000</a>
%F A210501 a(n) = A058339(n) - A210500(n).
%e A210501 The set {k: phi(k) = 12} is {13, 21, 26, 28, 36, 42}. Thus, if phi(k) = prime(6) - 1, the equation has exactly two odd solutions. Hence, a(6) = 2.
%t A210501 r = 87; lst1 = Table[EulerPhi[n], {n, (Prime[r] - 1)^2 + 1}]; lst2 = {}; Do[p = Prime[n]; AppendTo[lst2, Length@Select[Flatten@Position[Take[lst1, {p - 1, (p - 1)^2 + 1}], Prime[n] - 1], EvenQ]], {n, r}]; lst2
%Y A210501 Cf. A000010, A032446, A058339, A210500, A210502.
%K A210501 nonn
%O A210501 1,4
%A A210501 _Arkadiusz Wesolowski_, Jan 19 2013