OFFSET
1,1
COMMENTS
Francis and Ray call these numbers "deceptive primes".
Pseudoprimes to base 10, A005939, not divisible by 3. If k is in the sequence, then (10^k-1)/9 is in the sequence, by Steuerwald's theorem; see A005935. - Thomas Ordowski, Apr 10 2016
41041 is the first term that has four prime divisors. - Altug Alkan, Apr 10 2016
LINKS
Charles R Greathouse IV, Table of n, a(n) for n = 1..10000
R. Francis and T. Ray, The deceptive primes to 2.10^7, Missouri J. Math. Sci. 12 (2000), no. 3, 145-158.
MAPLE
select(t -> not isprime(t) and (10&^(t-1) - 1) mod (9*t) = 0, [seq(t, t=3..10^5, 2)]); # Robert Israel, Apr 10 2016
PROG
(PARI) p=5; forprime(q=7, 1e5, forstep(n=p+2, q-2, 2, if(n%5 && Mod(10, 9*n)^(n-1)==1, print1(n", "))); p=q) \\ Charles R Greathouse IV, Jul 31 2011
CROSSREFS
KEYWORD
nonn
AUTHOR
Tim Ray (c268scm(AT)semovm.semo.edu)
STATUS
approved