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A230030
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Numbers k not divisible by 5 such that k^2 written in base 5 has no digit > 1.
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5
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1, 972799, 3051273374, 6132750376, 839228035909, 3818357814376, 2384643515634376, 1490173338867234376, 931329727148437734376, 582077503203735352734376, 363797992467864990240234376
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OFFSET
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1,2
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COMMENTS
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If P(x) = 25x^4 + 15x^3 - 4x^2 + 3x + 1 then P(5^k) belongs to the sequence for every k > 2.
The initial condition is added to avoid trivial solutions of the form a(k)*5^m (m>0), whose square would always have the digits 1 and 0 in base 5. The previous subsequence of solutions P(5^k) consists of numbers written "10{k}24{k}10{k-1}30{k-1}1" in base 5, where "d{k}" means "digit d repeated k times". These terms (written in base 10) end in ...376. For k=8 this yields 582077503203735352734376 which might be the next term of the sequence. See A257283 and A257284 for the (less interesting) base 3 and base 4 analog. For the b=7 analog, the smallest nontrivial term is 20; for b=8 the first nontrivial terms are 3 and 11677. What are the subsequent terms, and the smallest nontrivial term for the b=6 analog? - M. F. Hasler, May 02 2015
Conjecture: a(k) = P(5^(k-2)) for every k > 5. - David Radcliffe, Sep 14 2018
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LINKS
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J. M. Borwein, Y. Bugeaud, and M. Coons, The legacy of Kurt Mahler, Notices of the American Mathematical Society, 62 5 (2015), 526-531.
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EXAMPLE
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972799 belongs to the sequence because 972799^2 = 111001100000110101111001100000110101 (base 5).
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PROG
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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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EXTENSIONS
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STATUS
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approved
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