A220349 - OEIS

A220349

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

2, 2, 3, 1, 7, 5, 8, 8, 60, 51, 43, 50, 388, 350, 322, 345, 2632, 2422, 2253, 2231, 18770, 17751, 17066, 16887, 141594, 134894, 130276, 128814, 1105560, 1058667, 1020020, 1007777, 8838825, 8485595, 8188908, 8106052

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OFFSET

1,1

COMMENTS

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.

LINKS

Table of n, a(n) for n=1..36.

EXAMPLE

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

CROSSREFS

Cf. A220344, A220248.

Sequence in context: A016539 A345886 A373880 * A273218 A173160 A022461

Adjacent sequences: A220346 A220347 A220348 * A220350 A220351 A220352

KEYWORD

nonn,base

AUTHOR

James G. Merickel, Dec 11 2012

STATUS

approved