A067519 - OEIS

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A067519

Numbers k such that k^2 + 1 is composite and phi(k^2 + 1) == 0 (mod k).

8, 32, 50, 64, 128, 192, 216, 336, 360, 900, 1000, 1152, 1250, 1280, 1344, 1408, 1800, 2450, 2880, 4096, 5440, 6528, 7564, 9216, 9800, 11008, 12544, 13824, 14450, 14976, 16576, 18432, 20000, 21420, 21440, 24200, 25675, 36000, 36288, 43778, 46656

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OFFSET

1,1

COMMENTS

If k^2 + 1 is prime, trivially phi(k^2 + 1) == 0 (mod k).

LINKS

Amiram Eldar, Table of n, a(n) for n = 1..500 (terms 1..300 from Donovan Johnson)

MATHEMATICA

n2Q[n_]:=Module[{c=n^2+1}, CompositeQ[c]&&Divisible[EulerPhi[c], n]]; Select[Range[50000], n2Q] (* Harvey P. Dale, Apr 16 2015 *)

PROG

(PARI) for(n=1, 46656, m=n^2+1; if(isprime(m)==0, if(eulerphi(m)%n==0, print1(n ", ")))) \\ Donovan Johnson, Nov 14 2013

CROSSREFS

Cf. A000010, A066820.

Sequence in context: A371454 A129749 A005879 * A253295 A290960 A009245

Adjacent sequences: A067516 A067517 A067518 * A067520 A067521 A067522

KEYWORD

nonn

AUTHOR

Benoit Cloitre, Feb 22 2002

STATUS

approved