OFFSET
0,2
COMMENTS
a(n) is the first odd number k for which k * 2^i + 1 is prime when i = n but composite for all i: 0 <= i < n, or 0 if no such k exists. Thus it is the first k for which A046067((k+1)/2) = n, and therefore also the first k for which you need to test the primality of exactly n values to show that it is not a Sierpiński number.
Jaeschke shows that for each n>0, the set {k : A046067((k+1)/2) = n} is infinite. - Jeppe Stig Nielsen, Jul 06 2020
LINKS
Hugo van der Sanden, Table of n, a(n) for n = 0..3253
G. Jaeschke, On the Smallest k Such that All k*2^N + 1 are Composite, Mathematics of Computation, Vol. 40, No. 161 (Jan., 1983), pp. 381-384.
EXAMPLE
7 * 2^i + 1 is composite for i < 2 (with values 8, 15) but prime for i = 2 (29); the smaller odd numbers 1, 3 and 5 each yield a prime for smaller i, so a(2) = 7.
PROG
(PARI) a(n)=forstep(k=1, +oo, 2, for(i=0, n-1, ispseudoprime(k<<i+1)&&next(2)); ispseudoprime(k<<n+1)&&return(k)) \\ Jeppe Stig Nielsen, Jul 06 2020
CROSSREFS
KEYWORD
nonn
AUTHOR
Hugo van der Sanden, Jul 23 2015
STATUS
approved