Consider numbers of the form ...31975319753197531, whose digits read from the right are 1,3,5,7,9,1,3,5,7,9,1,... Sequence gives lengths of these numbers which are primes.

A090743

Consider numbers of the form ...31975319753197531, whose digits read from the right are 1,3,5,7,9,1,3,5,7,9,1,... Sequence gives lengths of these numbers which are primes.