A131546 Least power of 3 having exactly n consecutive 7's in its decimal representation.

3, 11, 112, 184, 721, 3520, 6643, 12793, 67448, 208380, 364578, 1123485, 9549790, 23340555, 88637856

Offset

1,1

Comments

No more terms < 10^8. - Bert Dobbelaere, Mar 20 2019

Links

Table of n, a(n) for n=1..15.

Formula

a(1) = A063566(7). - Michel Marcus, Aug 05 2014

Example

a(2) = 11 because 3^11 = 177147 is the smallest power of 3 to contain a run of two consecutive 7's in its decimal form.

Mathematica

a = ""; Do[ a = StringJoin[a, "7"]; b = StringJoin[a, "7"]; k = 1; While[ StringPosition[ ToString[3^k], a] == {} || StringPosition[ ToString[3^k], b] != {}, k++ ]; Print[k], {n, 10} ]

Prog

(Python)
def A131546(n):
....str7 = '7'*n
....x, exponent = 3, 1
....while not str7 in str(x):
........exponent += 1
........x *= 3
....return exponent # Chai Wah Wu, Aug 05 2014

Crossrefs

Cf. A195269, A131552, A131551, A131550, A131549, A131548, A131547, A131545, A131544.

Sequence in context: A241587 A183381 A136985 * A275338 A068693 A036930

Adjacent sequences: A131543 A131544 A131545 * A131547 A131548 A131549

Keyword

more,nonn,base

Author

Shyam Sunder Gupta, Aug 26 2007

Extensions

a(11)-a(13) from Lars Blomberg, Feb 02 2013

a(14)-a(15) from Bert Dobbelaere, Mar 20 2019

Status

approved