login
A135358
Numbers n such that 7^n and 7^(n+1) have the same number of decimal digits.
2
6, 12, 19, 25, 32, 38, 45, 51, 58, 64, 71, 77, 83, 90, 96, 103, 109, 116, 122, 129, 135, 142, 148, 154, 161, 167, 174, 180, 187, 193, 200, 206, 213, 219, 225, 232, 238, 245, 251, 258, 264, 271, 277, 284, 290, 296, 303, 309, 316, 322, 329, 335, 342, 348, 355
OFFSET
1,1
COMMENTS
The linear recurrence of order 12 with signature (1,0,0,0,0,0,0,0,0,0,1,-1) is incorrect, it yields 509 instead of a(79) = 510. - Georg Fischer, Oct 25 2022
LINKS
EXAMPLE
a(1)=6 because {7^6, 7^7} = {117649, 823543} (6 digits),
a(2)=12 because {7^12,7^13}={13841287201,96889010407} (11 digits).
Sequence of first differences are only quasi-periodic:
6,7,6,7,6,7,6,7,6,7,6,6,7,
6,7,6,7,6,7,6,7,6,6,7,
6,7,6,7,6,7,6,7,6,6,7,
6,7,6,7,6,7,6,7,6,6,7,
6,7,6,7,6,7,6,7,6,6,7,
6,7,6,7,6,7,6,7,6,6,7,
6,7,6,7,6,7,6,7,6,7,6,6,7.
MATHEMATICA
Select[Range[100], IntegerLength[7^(#)] == IntegerLength[7^(# + 1)] &] (* G. C. Greubel, Oct 11 2016 *)
SequencePosition[IntegerLength/@(7^Range[400]), {x_, x_}][[All, 1]]((* Requires Mathematica version 10 or later *) * Harvey P. Dale, Feb 14 2020 *)
PROG
(PARI) isok(n) = #digits(7^n) == #digits(7^(n+1)); \\ Michel Marcus, Oct 13 2013
CROSSREFS
Sequence in context: A344177 A100357 A190265 * A187391 A081846 A078816
KEYWORD
base,nonn
AUTHOR
Zak Seidov, Dec 08 2007
STATUS
approved