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A220248 The number of palindromic primes leading and ending with 1, 3, 7 or 9 -- in sequence -- of length 2*ceiling(n/4)+1. 2
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 (list; graph; refs; listen; history; text; internal format)
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: A019179 A206568 A304656 * 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

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Last modified May 17 04:46 EDT 2022. Contains 353730 sequences. (Running on oeis4.)