A220344 The number of reversible primes (palindromic or emirps) by increasing permissible leading digit and by length.

1, 1, 1, 1, 3, 2, 3, 1, 12, 9, 12, 10, 60, 51, 43, 50, 413, 377, 346, 363, 2632, 2422, 2253, 2231, 18960, 17923, 17221, 17038, 141594, 134894, 130276, 128814, 1106984, 1059947, 1021263, 1009002, 8838825, 8485595, 8188908, 8106052

Offset

1,5

Comments

Aside from the first 4 terms here (corresponding to 2, 3, 5 and 7), the counts of 4 successive terms are of primes with leading digit 1, 3, 7 and 9 that are still prime if their decimal representations are reversed. A220248 and A220349 handle palindromic primes and emirps separately.

Links

Table of n, a(n) for n=1..40.

Example

a(5) is the start of the counts for 2-digit primes. 11, 13 and 17 are the only 2-digit primes with leading digit 1 that remain prime read backward.  It is not possible for a prime of more than 1 digit to lead with 2 and yield a prime read in reverse, so the following term, a(6), is the count for leading digit 3.

Crossrefs

Cf. A220248, A220349.

Sequence in context: A195381 A144558 A307551 * A176102 A344220 A318056

Adjacent sequences: A220341 A220342 A220343 * A220345 A220346 A220347

Keyword

nonn,base

Author

James G. Merickel, Dec 11 2012

Status

approved