A292931 Numbers n such that the sum of digits of 3^n (A004166) is divisible by 7.

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

Offset: 1

Author

Robert Israel, Sep 27 2017

Keywords

• nonn
• base

Examples

			a(3) = 30 is in the sequence because 3^30 = 205891132094649 has sum of digits 63, which is divisible by 7.
		

Links

• Chai Wah Wu, Table of n, a(n) for n = 1..10000
• Mathematics StackExchange, Is sum of digits of 3^1000 divisible by 7?

Crossrefs

Cf. A004166.

Programs

Maple

select(n -> convert(convert(3^n,base,10),`+`) mod 7 = 0, [$1..1000]);

Mathematica

Select[Range[600],Divisible[Total[IntegerDigits[3^#]],7]&] (* Harvey P. Dale, Mar 01 2018 *)

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