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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 (list; graph; refs; listen; history; text; internal format)
 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 Join[{2, 3, 4, 5, 6}, Select[Range[7, 250], Mod[# *DivisorSigma[1, #], EulerPhi[#]] == 2&]] (* Harvey P. Dale, Oct 28 2013 *) 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 Adjacent sequences:  A051945 A051946 A051947 * A051949 A051950 A051951 KEYWORD nonn,nice,easy AUTHOR N. J. A. Sloane, Dec 21 1999 STATUS approved

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Last modified June 16 16:09 EDT 2021. Contains 345063 sequences. (Running on oeis4.)