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A294652
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Positive integers k such that the sum of decimal digits of (4^k - 1) equals 3*k.
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1
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1, 2, 3, 4, 6, 7, 8, 10, 12, 13, 14, 20, 23, 24, 25, 26, 27, 28, 34, 36, 41, 46, 65, 71, 74, 83, 86, 89, 92, 111, 120, 235, 238, 253, 297, 366, 446
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OFFSET
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1,2
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COMMENTS
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No other terms below 10^6.
Conjecture: For k > 446, digsum(4^k - 1) < 3*k. This would mean that no further terms exist in the sequence.
If a(n) is even, then a(n)/2 is in A165722.
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LINKS
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EXAMPLE
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4^2 - 1 is 15 with sum of digits 6, so 2 is a term.
4^3 - 1 is 63 with sum of digits 9, so 3 is a term.
4^5 - 1 is 1023 with sum of digits 6, so 5 is not a term.
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MATHEMATICA
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Select[Range[2500], 3# == Total[IntegerDigits[4^# - 1]] &] (* G. C. Greubel, Nov 28 2017 *)
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PROG
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(PARI) is(n) = 3*n == sumdigits(4^n-1)
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CROSSREFS
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KEYWORD
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nonn,base,more
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AUTHOR
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STATUS
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approved
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