2,1

a(4) is M21, the 21st Mersenne prime with prime digit count of 2917

Cf. A028335.

Sequence in context: A119344 A072877 A032329 this_sequence A061490 A048827 A032077

Adjacent sequences: A066224 A066225 A066226 this_sequence A066228 A066229 A066230

nonn

Enoch Haga (Enokh(AT)comcast.net), Dec 18 2001