A063877 - OEIS

A063877

Composite numbers not divisible by 7 which in base 7 contain their largest proper factor as a substring.

551, 989, 1333, 1363, 5429, 20081, 26167, 28381, 28577, 29719, 60803, 139163, 147733, 150047, 161647, 217103, 224437, 262921, 293921, 298007, 302387, 312913, 342103, 345127, 385937, 436129, 508309, 509189, 539647, 829921, 873827, 921391

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OFFSET

1,1

LINKS

Table of n, a(n) for n=1..32.

MATHEMATICA

Do[ If[ !PrimeQ[ n ] && Mod[ n, 7 ] != 0 && StringPosition[ ToString[ FromDigits[ IntegerDigits[ n, 7 ] ] ], ToString[ FromDigits[ IntegerDigits[ Divisors[ n ][ [ -2 ] ], 7 ] ] ] ] != {}, Print[ n ] ], {n, 3, 10^6} ]

Select[Range[10^6], CompositeQ[#]&&Mod[#, 7]!=0&&SequenceCount[ IntegerDigits[ #, 7], IntegerDigits[Divisors[#][[-2]], 7]]>0&] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Mar 15 2020 *)

CROSSREFS

Sequence in context: A034282 A127347 A351664 * A372895 A204870 A263947

Adjacent sequences: A063874 A063875 A063876 * A063878 A063879 A063880

KEYWORD

base,nonn

AUTHOR

Robert G. Wilson v, Aug 27 2001

STATUS

approved