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A025726
Index of 7^n within the sequence of the numbers of the form 7^i*9^j.
1
1, 2, 4, 7, 11, 16, 22, 29, 37, 45, 54, 64, 75, 87, 100, 114, 129, 145, 161, 178, 196, 215, 235, 256, 278, 301, 325, 349, 374, 400, 427, 455, 484, 514, 545, 576, 608, 641, 675, 710, 746, 783, 821, 860, 899, 939, 980, 1022, 1065, 1109, 1154, 1200, 1247, 1294, 1342
OFFSET
0,2
COMMENTS
Positions of zeros in A025681. - R. J. Mathar, Jul 06 2025
LINKS
FORMULA
From Amiram Eldar, Nov 13 2025: (Start)
A025631(a(n)) = 7^n.
a(n) = 1 + Sum_{k=0..n} ceiling(k * c), where c = log_9(7) (A153619).
a(n) ~ c * n^2 / 2, where c is defined above. (End)
MATHEMATICA
Accumulate[Table[Ceiling[n * Log[9, 7]], {n, 0, 60}]] + 1 (* Amiram Eldar, Nov 13 2025 *)
CROSSREFS
KEYWORD
nonn,easy
EXTENSIONS
Offset corrected by Amiram Eldar, Nov 13 2025
STATUS
approved