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A007370 Numbers n such that sigma(x) = n has a unique solution.
(Formerly M2319)
20
1, 3, 4, 6, 7, 8, 13, 14, 15, 20, 28, 30, 36, 38, 39, 40, 44, 57, 62, 63, 68, 74, 78, 91, 93, 102, 110, 112, 121, 127, 133, 138, 150, 158, 160, 162, 164, 171, 174, 176, 183, 194, 195, 198, 200, 204, 212, 217, 222, 230, 242, 255, 256, 258, 260, 266, 278, 282 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

REFERENCES

M. Abramowitz and I. A. Stegun, eds., Handbook of Mathematical Functions, National Bureau of Standards Applied Math. Series 55, 1964 (and various reprintings), p. 840.

W. Sierpiński, Elementary Theory of Numbers. Państ. Wydaw. Nauk., Warsaw, 1964, p. 165.

N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

LINKS

T. D. Noe, Table of n, a(n) for n = 1..1000

M. Abramowitz and I. A. Stegun, eds., Handbook of Mathematical Functions, National Bureau of Standards, Applied Math. Series 55, Tenth Printing, 1972 [alternative scanned copy].

M. Abramowitz and I. A. Stegun, eds., Handbook of Mathematical Functions, National Bureau of Standards Applied Math. Series 55, 1964 (and various reprintings), p. 840.

W. Sierpiński, Elementary Theory of Numbers, Warszawa 1964.

R. G. Wilson, V, Letter to N. J. A. Sloane, Jul. 1992

MATHEMATICA

a = Table[ 0, {250} ]; Do[ s = DivisorSigma[ 1, n ]; If[ s < 251, a[ [ s ] ]++ ], {n, 1, 250} ]; Select[ Range[ 250 ], a[ [ # ] ] == 1 & ]

PROG

(PARI) list(lim)=my(v=vectorsmall(lim\1), u=List(), s); for(k=1, #v, s=sigma(k); if(s<=#v, v[s]++)); for(k=1, #v, if(v[k]==1, listput(u, k))); Vec(u) \\ Charles R Greathouse IV, Jun 15 2015

CROSSREFS

Cf. A007369, A007371, A007372, etc.

Sequence in context: A108348 A085149 A239458 * A322376 A044813 A154661

Adjacent sequences:  A007367 A007368 A007369 * A007371 A007372 A007373

KEYWORD

nonn

AUTHOR

N. J. A. Sloane, Mira Bernstein, Robert G. Wilson v

STATUS

approved

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Last modified October 13 18:57 EDT 2019. Contains 327981 sequences. (Running on oeis4.)