A167874 - OEIS

%I #5 Dec 03 2014 20:02:09

%S 3,15,69,409,2447,16337,117959,880623,6849011,54825357,448752095,

%T 3741170439,31669329743,271560643329,2354418484607,20608371394595,

%U 181897678706317,1617351777154871

%N The number of distinct primes < 10^n which are members of twin-prime pairs.

%C Number of terms in A001097 with at most n digits.

%C The entries are odd because 5 is counted only once, although it is a member of two twin prime pairs, (3,5) and (5,7).

%C Cardinality of the set of {A001359 U A006512} with entries < 10^n. - _R. J. Mathar_, Nov 23 2009

%F a(n) = 2*A007508(n) - 1.

%e There are exactly a(2) = 15 distinct primes below 10^2 that form twin primes (p, p+2), namely,

%e 3, 5, 7, 11, 13, 17, 19, 29, 31, 41, 43, 59, 61, 71, 73 :

%e (3, 5), (5, 7), (11, 13), (17, 19), (29, 31), (41, 43), (59, 61), (71, 73).

%Y Cf. A001097, A077800, A007508.

%K nonn

%O 1,1

%A _Lekraj Beedassy_, Nov 14 2009

%E Definition rephrased by _R. J. Mathar_, Nov 23 2009