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A292931
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Numbers n such that the sum of digits of 3^n (A004166) is divisible by 7.
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2
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25, 26, 30, 32, 47, 58, 79, 81, 87, 89, 102, 123, 141, 144, 145, 151, 164, 176, 178, 193, 201, 227, 239, 242, 257, 264, 282, 289, 300, 306, 319, 324, 329, 335, 336, 338, 348, 351, 358, 365, 395, 403, 437, 441, 450, 460, 468, 484, 489, 492, 495, 517, 518, 541, 542, 544, 554, 555, 563, 565, 570
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OFFSET
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1,1
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LINKS
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EXAMPLE
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a(3) = 30 is in the sequence because 3^30 = 205891132094649 has sum of digits 63, which is divisible by 7.
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MAPLE
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select(n -> convert(convert(3^n, base, 10), `+`) mod 7 = 0, [$1..1000]);
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MATHEMATICA
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Select[Range[600], Divisible[Total[IntegerDigits[3^#]], 7]&] (* Harvey P. Dale, Mar 01 2018 *)
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PROG
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(PARI) isok(n) = !(sumdigits(3^n) % 7); \\ Michel Marcus, Sep 27 2017
(Python)
from __future__ import division
A292931_list = [n for n in range(1000) if not sum(int(d) for d in str(3**n)) % 7] # Chai Wah Wu, Sep 28 2017
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CROSSREFS
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KEYWORD
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nonn,base
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AUTHOR
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STATUS
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approved
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