A069068 - OEIS

A069068

Composite numbers k+1 such that k*phi(k+1) is a perfect square.

9, 27, 50, 169, 242, 245, 945, 1127, 1280, 1352, 1694, 2430, 4335, 4410, 4624, 4802, 7500, 7935, 8624, 11913, 12544, 13254, 15050, 16384, 24642, 27869, 29403, 31827, 33489, 36750, 39675, 47045, 52800, 53290, 55470, 58806, 66248, 72075, 74060, 81675, 90774, 90972

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OFFSET

1,1

COMMENTS

Trivially if k+1 is prime k*phi(k+1) = k^2. If k>1 k*phi(k) is never a perfect square.

LINKS

Amiram Eldar, Table of n, a(n) for n = 1..1000

MATHEMATICA

Select[Range[60000], !PrimeQ[ #+1] && IntegerQ[Sqrt[ # EulerPhi[1+# ]]]&] (* T. D. Noe, Nov 01 2006 *)

CROSSREFS

Cf. A000010, A000290.

Sequence in context: A020208 A020306 A330680 * A051412 A027468 A158926