A060669 - OEIS

A060669

Numbers n such that phi(x) = n has exactly 6 solutions.

12, 16, 84, 88, 112, 232, 348, 408, 592, 736, 760, 780, 832, 952, 984, 1032, 1048, 1068, 1128, 1232, 1272, 1312, 1332, 1428, 1432, 1488, 1552, 1608, 1692, 1912, 2052, 2200, 2272, 2292, 2436, 2484, 2552, 2576, 2608, 2632, 2700, 2728, 2832, 2848, 3048, 3088

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OFFSET

1,1

LINKS

T. D. Noe, Table of n, a(n) for n = 1..1000

EXAMPLE

12 = phi(13) = phi(21) = phi(26) = phi(28) = phi(36) = phi(42).

MATHEMATICA

a = Table[ 0, {4000} ]; Do[ p = EulerPhi[ n ]; If[ p < 4001, a[ [ p ] ]++ ], {n, 1, 15000} ]; Select[ Range[ 4000 ], a[ [ # ] ] == 6 & ]

Take[Select[Tally[EulerPhi[Range[50000]]], #[[2]]==6&][[All, 1]]//Sort, 50] (* Harvey P. Dale, Sep 15 2016 *)

PROG

(PARI) is(n)=sum(i=1, n, eulerphi(i)==n)==6 \\ Charles R Greathouse IV, Mar 03 2014