OFFSET
1,1
COMMENTS
Conjecture: a(n)>1.
EXAMPLE
a(1)=3 because the following are four primes: 2*3-1, 2*3+1, 2*9-1, 2*9+1.
PROG
(Python)
from sympy import isprime
TOP=10**9
for p in range(2, 333):
if isprime(p):
failed = True
for x in range(1, TOP):
if isprime(p*x+1) and isprime(p*x-1) and isprime(p*x*x-1) and isprime(p*x*x+1):
print str(x)+', ',
failed = False
break
if failed: print '-1, ',
(PARI) a(n) = {k = 1; p = prime(n); while (! (isprime(p*k-1) && isprime(p*k+1) && isprime(p*k^2-1) && isprime(p*k^2+1)), k++); k; } \\ Michel Marcus, Dec 27 2015
CROSSREFS
KEYWORD
nonn
AUTHOR
Alex Ratushnyak, Dec 25 2015
STATUS
approved