A158725 Non-repdigit composite numbers not divisible by 2, 3, 5 or 11.

49, 91, 119, 133, 161, 169, 203, 217, 221, 247, 259, 287, 289, 299, 301, 323, 329, 343, 361, 371, 377, 391, 403, 413, 427, 437, 469, 481, 493, 497, 511, 527, 529, 533, 551, 553, 559, 581, 589, 611, 623, 629, 637, 667, 679, 689, 697, 703, 707, 713, 721, 731

Offset: 1

Author

Lekraj Beedassy, Mar 24 2009

Keywords

• nonn

Comments

Non-repdigit composite numbers ending in 1, 3, 7 or 9, with digital root not a multiple of 3 and whose alternate digit sums do not differ by a multiple of 11.

The "compositeness" of larger entries of the sequence is not obvious right away or deducible by mere inspection, and hence these terms readily lend themselves to be (erroneously) suspected as primes to the casual glance.

This differs from the corresponding sequence without the repunit condition starting at a(1351) = 11123 rather than 11111. - Charles R Greathouse IV, Sep 08 2012

Links

• Charles R Greathouse IV, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A007775, A053795, A008364.

Programs

PARI

is(n)=gcd(n,330)==1&&!isprime(n)&&n%(10^#Str(n)\9) \\ Charles R Greathouse IV, Sep 08 2012

Formula

a(n) ~ kn with k = 33/8. - Charles R Greathouse IV, Sep 08 2012

Extensions

Corrected and extended by Ray Chandler, Mar 27 2009