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A015949
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Numbers k such that k | 3^k + 1.
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23
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1, 2, 10, 50, 250, 1250, 5050, 6250, 11810, 25250, 31250, 59050, 126250, 156250, 295250, 510050, 631250, 750250, 781250, 1476250, 2125250, 2550250, 3156250, 3751250, 3906250, 5964050, 7381250, 10626250, 12751250, 13947610, 15781250
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OFFSET
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1,2
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COMMENTS
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a(n) mod 20 = 10 for n >= 3. - G. C. Greubel, Nov 05 2018
This sequence is infinite, because for n > 1, 3^a(n) + 1 is in this sequence. - Jinyuan Wang, Nov 06 2018
For the provided data, if k is a term then p*k is a term where p is an odd divisor of k. - David A. Corneth, Nov 06 2018
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LINKS
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Giovanni Resta, Table of n, a(n) for n = 1..180 (first 100 terms from G. C. Greubel)
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MATHEMATICA
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Do[If[PowerMod[3, n, n] + 1 == n, Print[n]], {n, 1, 10^7}] (* Jinyuan Wang, Nov 01 2018 *)
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PROG
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(PARI) for(n=1, 10^7, if(Mod(3, n)^n==-1, print1(n, ", "))) \\ Jinyuan Wang, Nov 01 2018
(MAGMA) [n: n in [1..2*10^7]| Modexp(3, n, n)+1 eq n]; // Vincenzo Librandi, Nov 01 2018
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CROSSREFS
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Cf. A034472 (3^n+1).
Cf. A006521 (k | 2^k + 1), A015950 (k | 4^k + 1), A015951 (k | 5^k + 1).
Column k=3 of A333429.
Sequence in context: A180266 A015945 A015954 * A020699 A020729 A110170
Adjacent sequences: A015946 A015947 A015948 * A015950 A015951 A015952
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KEYWORD
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nonn
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AUTHOR
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Robert G. Wilson v
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EXTENSIONS
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Corrected by David W. Wilson
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STATUS
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approved
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