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A276037
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Numbers using only digits 1 and 5.
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16
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1, 5, 11, 15, 51, 55, 111, 115, 151, 155, 511, 515, 551, 555, 1111, 1115, 1151, 1155, 1511, 1515, 1551, 1555, 5111, 5115, 5151, 5155, 5511, 5515, 5551, 5555, 11111, 11115, 11151, 11155, 11511, 11515, 11551, 11555, 15111, 15115, 15151, 15155, 15511, 15515
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OFFSET
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1,2
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COMMENTS
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Numbers n such that product of digits of n is a power of 5.
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LINKS
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FORMULA
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a(2n+1) = 10 a(n) + 1.
a(2n+2) = 10 a(n) + 5.
G.f. g(x) satisfies g(x) = 10 (x + x^2) g(x^2) + (x + 5 x^2)/(1 - x^2). (End)
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EXAMPLE
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5551 is in the sequence because all of its digits are 1 or 5 and consequently because the product of digits, 5*5*5*1 = 125 = 5^3 is a power of 5.
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MAPLE
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S[0]:= [0]:
for d from 1 to 6 do S[d]:= map(t -> (10*t+1, 10*t+5), S[d-1]) od:
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MATHEMATICA
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Select[Range[20000], IntegerQ[Log[5, Times@@(IntegerDigits[#])]]&]
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PROG
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(Python)
from itertools import product
A276037_list = [int(''.join(d)) for l in range(1, 10) for d in product('15', repeat=l)] # Chai Wah Wu, Aug 18 2016
(Magma) [n: n in [1..20000] | Set(Intseq(n)) subset {1, 5}]; // Vincenzo Librandi, Aug 19 2016
(PARI) a(n) = my(v=[1, 5], b=binary(n+1), d=vector(#b-1, i, v[b[i+1]+1])); sum(i=1, #d, d[i] * 10^(#d-i)) \\ David A. Corneth, Aug 22 2016
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CROSSREFS
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Cf. numbers n such that product of digits of n is a power of k: A028846 (k=2), A174813 (k=3), this sequence (k=5), A276038 (k=6), A276039 (k=7).
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KEYWORD
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nonn,base
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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