A220248 The number of palindromic primes leading and ending with 1, 3, 7 or 9 -- in sequence -- of length 2*ceiling(n/4)+1.

5, 4, 4, 2, 25, 27, 24, 18, 190, 172, 155, 151, 1424, 1280, 1243, 1225, 10924, 10512, 10399, 10207, 92015, 88693, 87149, 85844, 788498, 767916, 744036, 736193, 6891972, 6755263, 6698063, 6699928, 61960057, 60731724, 58734513, 57667571, 552358972, 540945484, 533119350, 531667127

Offset

1,1

Comments

The way the sequence is titled/written is to give uniformity to the numbers considered. Thus, a(4k-3), a(4k-2), a(4k-1) and a(4k) are the counts of the (2k+1)-digit palprimes beginning with 1, 3, 7 and 9, respectively, leaving out consideration of the primes 2, 3, 5, 7 and 11 (the first 5 terms of A002385) as not really of type as far as this sequence is concerned.

Links

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

Example

If one wants, say, the number of 13-digit palindromic primes with 7 as the leading digit, then what one wants is the 3rd term after 5 groups of 4, or the 23rd, which is a(23)=87149.  The palprimes corresponding to a(1)=5 are 101, 131, 151, 181 and 191.

Crossrefs

Cf. A002385, A220344, A220349.

Sequence in context: A361588 A304656 A354209 * A147533 A241183 A137240

Adjacent sequences: A220245 A220246 A220247 * A220249 A220250 A220251

Keyword

nonn,base

Author

James G. Merickel, Dec 08 2012

Extensions

a(37)-a(40) added by James G. Merickel, Dec 30 2012

Status

approved