1,1

The sequence is finite because there is only one 4-digit prime from which no further 8-digit primes can be obtained

as there are no 4-digit partners left.

Start from the 4 single-digit primes 2,3,5,7. Construct all 2-digit primes

with prime digits (to get the first 8 elements of A019546). Add

the 4-digit primes obtained by concatenation of the previous 2-digit primes.

a={2, 3, 5, 7} (* two digits primes*) b=Delete[Union[Flatten[Table[If[PrimeQ[a[[n]]*10+a[[m]]]==True, a[[n]]*10+a[[m]], 0], {n, 1, 4}, {m, 1, 4}], 1]], 1] (* only one four digit prime combination exists*) c=Delete[Union[Flatten[Table[If[PrimeQ[b[[n]]*100+b[[m]]]==True, b[[n]]*100+b[[m]], 0], {n, 1, 4}, {m, 1, 4}], 1]], 1] (* no eight digit prime of this type exists*) d=If[PrimeQ[c[[1]]*10000+c[[1]]]==True, c[[1]]*100+c[[1]], 0] Join[a, b, c]

Sequence in context: A124674 A020994 A085823 this_sequence A155873 A106711 A048398

Adjacent sequences: A100549 A100550 A100551 this_sequence A100553 A100554 A100555

nonn,base,fini,full,less

Roger L. Bagula (rlbagulatftn(AT)yahoo.com), Nov 27 2004

Edited by the associate editors of the OEIS, Jun 28 2009