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A128246 Number of twin prime members occurring in the sequence of prime-index-primes for indices up to 10^n. 0
8, 37, 250, 2033, 17119, 144795 (list; graph; refs; listen; history; internal format)
OFFSET

1,1

COMMENTS

The ratio of successive terms appears to converge. They are as follows for n = 2 to 6:

n pip(10^n)/pip(10^(n-1))

-- ----------------------------

2 4.625000000000000000000000000

3 6.756756756756756756756756757

4 8.132000000000000000000000000

5 8.420560747663551401869158878

6 8.458145919738302470938723056

FORMULA

Here prime(n) is the n-th prime number, pip(n) = prime(prime(n)) is the n-th prime-index-prime.

EXAMPLE

The first 10^1 prime-index-primes are 3,5,11,17,31,41,59,67,83,109. The 8 primes 3,5,11,17,31,41,59,109 are also twin prime members. So 8 is the first entry in the table.

PROG

(PARI) g(n) = c=0; for(x=1, n, y=prime(prime(x)); if(isprime(y+2)||isprime(y-2), c++)); c

CROSSREFS

Sequence in context: A055799 A035038 A111645 * A204076 A163832 A139798

Adjacent sequences:  A128243 A128244 A128245 * A128247 A128248 A128249

KEYWORD

more,nonn

AUTHOR

Cino Hilliard (hillcino368(AT)hotmail.com), May 09 2007

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Last modified February 17 04:58 EST 2012. Contains 205985 sequences.