A256351 Composites in base 10 that remain composite in exactly seven bases b, 2 <= b <= 10, expansions interpreted as decimal numbers.

8, 9, 15, 16, 21, 22, 25, 28, 34, 75, 87, 91, 93, 94, 106, 111, 123, 141, 143, 145, 147, 155, 172, 201, 205, 214, 217, 237, 255, 298, 304, 305, 363, 371, 376, 377, 385, 388, 395, 403, 411, 423, 428, 442, 458, 466, 471, 473, 483, 495, 501, 505, 507, 531, 533

Offset

1,1

Links

Sebastian Petzelberger, Table of n, a(n) for n = 1..10000

Carlos Rivera, PP&P Puzzle 24: Primes in several bases

Maple

f:= proc(b, x) local L, i;
L:= convert(x, base, b);
isprime(add(10^(i-1)*L[i], i=1..nops(L)))
end proc:
select(t -> not isprime(t) and nops(select(f, [$2..9], t))=2, [$1..1000]); # Robert Israel, Mar 26 2015

Mathematica

fQ[n_] := CompositeQ@ n && Count[ CompositeQ[ FromDigits[ IntegerDigits[n, #]] & /@ Range[2, 9]], True] == 6; Select[ Range@ 500, fQ] (* Robert G. Wilson v, Mar 26 2015 *)

Crossrefs

Cf. A052026, A256350 - A256356.

Cf. A038537, A052027, A052028, A052029, A052030, A052031, A052032, A084482.

Cf. A052033, A236356.

Sequence in context: A114305 A101765 A306490 * A050688 A134334 A351048

Adjacent sequences: A256348 A256349 A256350 * A256352 A256353 A256354

Keyword

nonn,base

Author

Sebastian Petzelberger, Mar 25 2015

Status

approved