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

0, 2, 3, 5, 6, 14, 18, 20, 22, 28, 38, 40, 42, 62, 66, 68, 74, 76, 78, 82, 98, 102, 104, 106, 114, 124, 126, 136, 148, 156, 178, 194, 196, 200, 204, 208, 226, 242, 246, 248, 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

Offset

1,2

Links

Amiram Eldar, Table of n, a(n) for n = 1..15628 (terms below 10^5; terms 1..316 from Robert Israel)

Example

6 = cototient(10) and for all x<>10, cototient(x) <> 6.

Maple

N:= 1000: # for all terms <= N
cotot:= n -> n - numtheory:-phi(n):
V:= Vector(N):
for n from 2 to N^2 do
  v:= cotot(n);
  if v  > N then next fi;
  V[v]:= V[v]+1;
od:
0, op(select(t -> V[t]=1, [$1..N])); # Robert Israel, May 26 2019

Prog

(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

Crossrefs

Cf. A051953, A063742, A131826.

Sequence in context: A078203 A285937 A253647 * A289124 A304297 A249752

Adjacent sequences: A131822 A131823 A131824 * A131826 A131827 A131828

Keyword

nonn

Author

Franz Vrabec, Jul 19 2007

Extensions

More terms from Robert Israel, May 26 2019

Status

approved