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A025710
Index of 5^n within the sequence of the numbers of the form 5^i*9^j.
1
1, 2, 4, 7, 10, 14, 19, 25, 31, 38, 46, 55, 64, 74, 85, 96, 108, 121, 135, 149, 164, 180, 197, 214, 232, 251, 271, 291, 312, 334, 356, 379, 403, 428, 453, 479, 506, 534, 562, 591, 621, 652, 683, 715, 748, 781, 815, 850, 886, 922, 959, 997, 1036, 1075, 1115, 1156, 1198
OFFSET
0,2
COMMENTS
Positions of zeros in A025679. - R. J. Mathar, Jul 06 2025
LINKS
FORMULA
From Amiram Eldar, Nov 13 2025: (Start)
A025624(a(n)) = 5^n.
a(n) = 1 + Sum_{k=0..n} ceiling(k * c), where c = log_9(5) (A153205).
a(n) ~ c * n^2 / 2, where c is defined above. (End)
MATHEMATICA
Accumulate[Table[Ceiling[n * Log[9, 5]], {n, 0, 60}]] + 1 (* Amiram Eldar, Nov 13 2025 *)
CROSSREFS
KEYWORD
nonn,easy
EXTENSIONS
Offset corrected by Amiram Eldar, Nov 13 2025
STATUS
approved