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A051948
Numbers k such that k*sigma(k) == 2 (mod phi(k)).
1
2, 3, 4, 5, 6, 7, 11, 13, 17, 19, 22, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 101, 103, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 167, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227, 229, 233, 239, 241
OFFSET
1,1
REFERENCES
R. K. Guy, Unsolved Problems Number Theory, B37.
LINKS
Chris Nash, There no other solutions besides 4, 6, 22 and the primes (html, pdf)
C. Rivera, Puzzle 76
M. V. Subbarao, On two congruences for primality, Pacific Journal of Mathematics, 52 (1974), 261-268. (PDF)
MATHEMATICA
Select[Range[2, 250], Mod[#*DivisorSigma[1, #]-2, EulerPhi[#]]==0&] (* Harvey P. Dale, Sep 22 2021 *)
PROG
(PARI) a(n)=if(n<12, [2, 3, 4, 5, 6, 7, 11, 13, 17, 19, 22][n], prime(n-3)) \\ Charles R Greathouse IV, Aug 26 2011
CROSSREFS
Cf. A002270.
Sequence in context: A090563 A045745 A054010 * A193461 A192587 A174323
KEYWORD
nonn,nice,easy
AUTHOR
N. J. A. Sloane, Dec 21 1999
STATUS
approved