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A025693
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Index of 2^n within sequence of numbers of form 2^i*7^j.
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1
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1, 2, 3, 5, 7, 9, 12, 15, 18, 22, 26, 30, 35, 40, 45, 51, 57, 64, 71, 78, 86, 94, 102, 111, 120, 129, 139, 149, 159, 170, 181, 193, 205, 217, 230, 243, 256, 270, 284, 298, 313, 328, 343, 359, 375, 392, 409, 426, 444, 462, 480, 499, 518, 537, 557, 577, 597, 618, 639, 661, 683
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listen;
history;
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internal format)
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OFFSET
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0,2
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LINKS
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Charles R Greathouse IV, Table of n, a(n) for n = 0..10000
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FORMULA
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a(n) ~ kn^2 + O(n) with k = log(7)/log(2) - log(7)^2/log(2)^2. - Charles R Greathouse IV, Jun 28 2011
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PROG
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(PARI) a(n)=my(N=7<<n); sum(k=0, log(N)\log(7)-1, #binary(N\=7)) \\ Charles R Greathouse IV, Jun 28 2011
(PARI) a(n)=my(N=1); n+1+sum(i=1, n, logint(N<<=1, 7)); \\ Charles R Greathouse IV, Jan 11 2018
(PARI) first(n)=my(s, N=1/2); vector(n+1, i, s+=logint(N<<=1, 7)+1) \\ Charles R Greathouse IV, Jan 11 2018
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CROSSREFS
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Cf. A003591.
Sequence in context: A130518 A001840 A022794 * A117930 A090632 A022786
Adjacent sequences: A025690 A025691 A025692 * A025694 A025695 A025696
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KEYWORD
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nonn,easy
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AUTHOR
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David W. Wilson
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EXTENSIONS
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An incorrect g.f. was deleted by N. J. A. Sloane, Sep 16 2009
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STATUS
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approved
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