A220349 - OEIS

%I #11 Nov 17 2019 01:20:02

%S 2,2,3,1,7,5,8,8,60,51,43,50,388,350,322,345,2632,2422,2253,2231,

%T 18770,17751,17066,16887,141594,134894,130276,128814,1105560,1058667,

%U 1020020,1007777,8838825,8485595,8188908,8106052

%N Number of emirps of length ceiling(n/4)+1 and leading digit 1, 3, 7 or 9 (in sequence).

%C Beginning with lengths of 2 digits, each term represents the number of emirps -- reversible primes in base 10 that are not palindromic -- that begin with one of the permissible digits 1, 3, 7 or 9.  For odd lengths, it is derived in relationship to the sequences A220344 and A220248, the numbers of all reversibles by length and leading digit and the numbers of just palprimes (these sequences are not parallel by index, however).  For even lengths there are no palindromic primes -- with the sole exception of 11 -- so half of the terms are simply carried over from A220344.

%e The 2-digit emirps with leading digit 1 are 13 and 17 (as 31 and 71 are prime), so a(1)=2.

%Y Cf. A220344, A220248.

%K nonn,base

%O 1,1

%A _James G. Merickel_, Dec 11 2012