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A220344 The number of reversible primes (palindromic or emirps) by increasing permissible leading digit and by length. 2
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 (list; graph; refs; listen; history; text; internal format)
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 A318056 A096839

Adjacent sequences:  A220341 A220342 A220343 * A220345 A220346 A220347

KEYWORD

nonn,base

AUTHOR

James G. Merickel, Dec 11 2012

STATUS

approved

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Last modified December 13 06:26 EST 2019. Contains 329968 sequences. (Running on oeis4.)