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A102820 Number of primes between 2*prime(n) and 2*prime(n+1), where prime(n) is the n-th prime. 8
1, 1, 2, 2, 1, 2, 1, 2, 2, 2, 3, 1, 1, 1, 3, 3, 0, 2, 2, 0, 3, 1, 2, 4, 2, 0, 1, 0, 1, 6, 1, 3, 1, 3, 0, 3, 1, 1, 1, 3, 1, 3, 1, 1, 1, 4, 4, 1, 1, 2, 1, 1, 3, 2, 2, 0, 1, 1, 1, 1, 3, 6, 2, 0, 1, 6, 1, 3, 0, 1, 1, 3, 2, 2, 1, 2, 1, 1, 2, 4, 1, 3, 1, 1, 2, 1, 2, 1, 0, 1, 4, 2, 1, 3, 0, 2, 5, 0, 5, 3, 3, 2, 1, 0, 2 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

Number of primes between successive even semiprimes. [Juri-Stepan Gerasimov, May 01 2010]

LINKS

Reinhard Zumkeller, Table of n, a(n) for n = 1..10000

V. Shevelev, On critical small intervals containing primes, arXiv:0908.2319 [math.NT], 2009.

EXAMPLE

a(15)=3 because there are 3 primes between the doubles of the 15th and 16th primes, that is between 2*47 and 2*53.

MATHEMATICA

Table[PrimePi[2 Prime[n+1]]-PrimePi[2 Prime[n]], {n, 150}] (* Zak Seidov *)

PROG

(Haskell)

a102820 n = a102820_list !! (n-1)

a102820_list =  map (sum . (map a010051)) $

   zipWith enumFromTo a100484_list (tail a100484_list)

-- Reinhard Zumkeller, Apr 29 2012

(PARI) a(n) = primepi(2*prime(n+1)) - primepi(2*prime(n)); \\ Michel Marcus, Sep 22 2017

CROSSREFS

Cf. A104380.

Cf. A104272, A080359. [Vladimir Shevelev, Aug 24 2009]

Cf. A100484, A010051.

Sequence in context: A003643 A092788 A058062 * A024317 A024880 A029424

Adjacent sequences:  A102817 A102818 A102819 * A102821 A102822 A102823

KEYWORD

easy,nonn

AUTHOR

Ali A. Tanara (tanara(AT)khayam.ut.ac.ir), Feb 27 2005

EXTENSIONS

More terms from Zak Seidov, Feb 28 2005

STATUS

approved

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Last modified May 14 22:40 EDT 2021. Contains 343909 sequences. (Running on oeis4.)