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A236693 Numbers n such that 2^sigma(n) == 1 (mod n). 1
1, 3, 15, 35, 51, 65, 105, 119, 195, 255, 315, 323, 357, 377, 455, 459, 585, 595, 663, 969, 1045, 1071, 1105, 1131, 1189, 1365, 1455, 1469, 1485, 1547, 1615, 1785, 1799, 1885, 1887, 1911, 2261, 2295, 2385, 2639, 2795, 2907, 3135, 3145, 3185, 3213, 3315, 3339 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

LINKS

Table of n, a(n) for n=1..48.

Florian Luca, Positive integers n such that  n| a^sigma(n) - 1, Novi Sad Journal of Mathematics, Vol 33, No. 2 (2003), pp. 49-66.

EXAMPLE

2^sigma(15) = 2^24 = 16777216 is congruent to 1 (mod 15), so 15 is a term of the sequence.

MATHEMATICA

l = {1};

For[i = 1, i <= 10^4, i++,

    If[Mod[2^DivisorSigma[1, i], i] == 1, l = Append[l, i]]];

l

PROG

(PARI) s=[1]; for(n=1, 10000, if(2^sigma(n)%n==1, s=concat(s, n))); s \\ Colin Barker, Jan 30 2014

(PARI) isok(n) = Mod(2, n)^sigma(n)==1; \\ Altug Alkan, Sep 19 2017

CROSSREFS

Cf. A000203, A276238.

Sequence in context: A009261 A102790 A317182 * A317183 A000466 A241237

Adjacent sequences:  A236690 A236691 A236692 * A236694 A236695 A236696

KEYWORD

nonn,easy

AUTHOR

Joseph L. Pe, Jan 30 2014

EXTENSIONS

a(1) = 1 added by Amiram Eldar, Sep 19 2017

STATUS

approved

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Last modified December 15 20:00 EST 2019. Contains 330000 sequences. (Running on oeis4.)