A131827 Numbers k such that cototient(x) = k has exactly 2 solutions.

4, 7, 9, 11, 13, 15, 36, 37, 44, 46, 54, 56, 70, 80, 84, 88, 90, 92, 94, 112, 118, 138, 142, 152, 158, 160, 162, 164, 166, 174, 176, 182, 184, 188, 198, 210, 212, 214, 228, 230, 234, 236, 252, 272, 276, 278, 282, 304, 312, 316, 318, 320, 322, 328, 352, 354, 364

Offset

1,1

Links

Table of n, a(n) for n=1..57.

Example

4 = cototient(6) = cototient(8) and there are no other solutions.

Prog

(PARI) lista(nn) = {my(v=vector(nn^2, i, i - eulerphi(i))); for(k=0, nn, if(sum(i=1, k*k, k==v[i])==2, print1(k, ", "))); } \\ Jinyuan Wang, Mar 21 2020

Crossrefs

Cf. A051953, A063742, A131825, A131826.

Sequence in context: A310943 A310944 A024704 * A035245 A243191 A260647

Adjacent sequences: A131824 A131825 A131826 * A131828 A131829 A131830

Keyword

nonn

Author

Franz Vrabec, Jul 19 2007

Extensions

More terms from Jinyuan Wang, Mar 21 2020

Status

approved