A049283 a(n) is the smallest k such that phi(k)=n, where phi is Euler's totient function.

1, 3, 0, 5, 0, 7, 0, 15, 0, 11, 0, 13, 0, 0, 0, 17, 0, 19, 0, 25, 0, 23, 0, 35, 0, 0, 0, 29, 0, 31, 0, 51, 0, 0, 0, 37, 0, 0, 0, 41, 0, 43, 0, 69, 0, 47, 0, 65, 0, 0, 0, 53, 0, 81, 0, 87, 0, 59, 0, 61, 0, 0, 0, 85, 0, 67, 0, 0, 0, 71, 0, 73, 0, 0, 0, 0, 0, 79, 0, 123, 0, 83, 0, 129, 0, 0, 0, 89

Offset

1,2

Links

T. D. Noe, Table of n, a(n) for n = 1..10000

Example

The smallest k such that phi(k)=2 is k=3, so a(2)=3.

Mathematica

Module[{nn=140, ep}, ep=Table[{k, EulerPhi[k]}, {k, 0, nn}]; Table[SelectFirst[ep, #[[2]]==n&], {n, nn}]][[;; , 1]]/."NotFound"->0 (* Harvey P. Dale, Jul 29 2023 *)

Prog

(PARI) a(n)=if(n>2, for(k=n+1, solve(x=n, 2*n^2, x/(exp(Euler)*log(log(x))+3/log(log(x)))-n), if(eulerphi(k)==n, return(k))); 0, 2*n-1) \\ Charles R Greathouse IV, Nov 28 2012
(PARI) x=1000; v=vector(x\(exp(Euler)*log(log(x))+3/log(log(x)))); for(n=1, x, t=eulerphi(n); if(t<=#v && !v[t], v[t]=n)); v \\ Charles R Greathouse IV, Nov 28 2012

Crossrefs

Cf. A000010, A014197.

Sequence in context: A352505 A210524 A325961 * A141162 A160035 A281648

Adjacent sequences: A049280 A049281 A049282 * A049284 A049285 A049286

Keyword

nonn

Author

Jud McCranie, Oct 10 2000

Status

approved