1,3

Do[ If[ PrimeQ[ (10*7^n -1)/3 ], Print[ n ] ], {n, 0, 2500} ]

Cf. A057437.

Sequence in context: A031079 A157023 A101554 this_sequence A038341 A075718 A093794

Adjacent sequences: A057015 A057016 A057017 this_sequence A057019 A057020 A057021

nonn

Robert G. Wilson v (rgwv(AT)rgwv.com), Sep 09 2000