OFFSET
1,2
COMMENTS
Conjecture: numbers n such that n-1 is not a prime number. - Vincenzo Librandi, Jul 16 2016
Counterexamples to conjecture: 56, 306, 552, 870, ... are not in the sequence. These are p^2+p where p is prime but p^2+p-1 is not prime. - Robert Israel, Jul 04 2017
LINKS
Robert Israel, Table of n, a(n) for n = 1..10000
EXAMPLE
56 is not in the sequence because 56 - 56^2/8^2 = 7 is prime for k = 8.
MAPLE
filter:= n -> andmap(t -> not isprime(n - t^2), numtheory:-divisors(n)):
select(filter, [$1..1000]); # Robert Israel, Jul 04 2017
PROG
(PARI) isOK(n) = fordiv(n, k, if(isprime(n-(n/k)^2), return(0))); 1
s=[]; for(n=1, 100, if(isOK(n), s=concat(s, n))); s \\ Colin Barker, May 16 2014
CROSSREFS
KEYWORD
nonn
AUTHOR
Ilya Lopatin and Juri-Stepan Gerasimov, Apr 30 2014
STATUS
approved