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A256351
Composites in base 10 that remain composite in exactly seven bases b, 2 <= b <= 10, expansions interpreted as decimal numbers.
3
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
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 *)
KEYWORD
nonn,base
AUTHOR
STATUS
approved