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A131825 Numbers k such that cototient(x) = k has exactly 1 solution.

%I A131825 #18 Apr 08 2023 05:01:38
%S A131825 0,2,3,5,6,14,18,20,22,28,38,40,42,62,66,68,74,76,78,82,98,102,104,
%T A131825 106,114,124,126,136,148,156,178,194,196,200,204,208,226,242,246,248,
%U A131825 254,258,262,296,302,306,308,314,330,338,342,356,370,374,378,388,398,400,408,416,418,422,426,434,438
%N A131825 Numbers k such that cototient(x) = k has exactly 1 solution.
%H A131825 Amiram Eldar, <a href="/A131825/b131825.txt">Table of n, a(n) for n = 1..15628</a> (terms below 10^5; terms 1..316 from Robert Israel)
%e A131825 6 = cototient(10) and for all x<>10, cototient(x) <> 6.
%p A131825 N:= 1000: # for all terms <= N
%p A131825 cotot:= n -> n - numtheory:-phi(n):
%p A131825 V:= Vector(N):
%p A131825 for n from 2 to N^2 do
%p A131825   v:= cotot(n);
%p A131825   if v  > N then next fi;
%p A131825   V[v]:= V[v]+1;
%p A131825 od:
%p A131825 0, op(select(t -> V[t]=1, [$1..N])); # _Robert Israel_, May 26 2019
%o A131825 (PARI) lista(nn) = {my(v=vector(nn^2+2, i, i - eulerphi(i))); for(k=0, nn, if(sum(i=1, k*k+2, k==v[i])==1, print1(k, ", "))); } \\ _Jinyuan Wang_, Mar 21 2020
%Y A131825 Cf. A051953, A063742, A131826.
%K A131825 nonn
%O A131825 1,2
%A A131825 _Franz Vrabec_, Jul 19 2007
%E A131825 More terms from _Robert Israel_, May 26 2019