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A051948 Numbers n such that n*sigma(n) == 2 mod phi(n). 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

Table of n, a(n) for n=1..56.

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 August 22 20:47 EDT 2019. Contains 326209 sequences. (Running on oeis4.)