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A328375
Numbers k such that the decimal expansion of 2^k contains the substring 777.
1
24, 40, 75, 152, 166, 179, 181, 191, 194, 199, 214, 230, 235, 260, 282, 296, 304, 311, 317, 323, 326, 332, 342, 345, 363, 370, 374, 390, 417, 424, 426, 443, 455, 468, 471, 474, 475, 483, 489, 490, 505, 512, 523, 524, 536, 540, 559, 567, 581, 584, 585, 588, 593
OFFSET
1,1
COMMENTS
The decimal expansion of 2^k ends in 7776 iff k == 40 (mod 500), so the sequence is infinite. - Jon E. Schoenfield, Oct 14 2019
Conjecture: if n > 30536, then a(n) = n + 3623. - Chai Wah Wu, Oct 26 2019
EXAMPLE
16777216 = 2^24.
MAPLE
q:= n-> searchtext("777", cat(2^n))>0:
select(q, [$1..600])[]; # Alois P. Heinz, Oct 26 2019
MATHEMATICA
aQ[n_] := SequenceCount[IntegerDigits[2^n], {7, 7, 7}] > 0; Select[Range[660], aQ] (* Amiram Eldar, Oct 26 2019 *)
PROG
(Python)
A328375_list = [k for k in range(1000) if '777' in str(2**k)] # Chai Wah Wu, Oct 26 2019
CROSSREFS
Cf. A007356 (contains 666), A030000 (contains n).
Sequence in context: A211567 A316271 A357699 * A141717 A185742 A286876
KEYWORD
nonn,easy,base
AUTHOR
Eder Vanzei, Oct 14 2019
STATUS
approved