

A103960


Number of primes p such that prime(n)*p  2 is prime and p <= prime(n).


4



1, 1, 2, 2, 2, 3, 1, 2, 4, 3, 2, 5, 3, 3, 5, 3, 4, 4, 4, 5, 6, 3, 4, 6, 3, 5, 6, 5, 3, 6, 5, 8, 5, 5, 6, 5, 7, 9, 7, 6, 6, 5, 7, 6, 8, 5, 8, 10, 5, 6, 8, 6, 7, 7, 10, 10, 3, 11, 8, 11, 6, 10, 8, 12, 9, 9, 7, 11, 9, 7, 8, 9, 6, 14, 8, 10, 11, 11, 12, 11, 7, 8
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OFFSET

1,3


COMMENTS

Conjecture: All items of this sequence are greater than or equal to 1. Tested to prime(1000000).
a(A137291(n)) = A210481(A137291(n)) + 1.  Reinhard Zumkeller, Jul 30 2015


LINKS

T. D. Noe, Table of n, a(n) for n = 1..1000


EXAMPLE

Prime(1)*22 = 2, so a(1)=1;
Prime(3) = 5, 5*32 = 13, 5*52 = 23, so a(3)=2;


MATHEMATICA

Table[p=Prime[n]; ct=0; Do[pk=Prime[k]; If[PrimeQ[p*pk2], ct=ct+1], {k, n}]; ct, {n, 100}]


PROG

(Haskell)
a103960 n = sum [a010051' $ p * q  2 
let p = a000040 n, q < takeWhile (<= p) a000040_list]
 Reinhard Zumkeller, Jul 30 2015


CROSSREFS

Cf. A103959.
Cf. A010051, A000040, A137291, A210481.
Sequence in context: A256915 A209254 A227738 * A242626 A240689 A233567
Adjacent sequences: A103957 A103958 A103959 * A103961 A103962 A103963


KEYWORD

easy,nonn


AUTHOR

Lei Zhou, Feb 22 2005


STATUS

approved



