

A322006


a(n) = number of primes of the form p = n  q, where q is a prime or semiprime.


2



0, 0, 0, 0, 1, 2, 2, 3, 3, 4, 3, 3, 4, 4, 4, 4, 6, 5, 5, 4, 6, 5, 7, 4, 8, 5, 8, 5, 9, 4, 7, 4, 8, 7, 9, 4, 11, 5, 9, 6, 11, 6, 11, 6, 11, 8, 12, 4, 13, 6, 12, 8, 13, 6, 14, 5, 13, 8, 13, 4, 16, 5, 15, 9, 16, 7, 16, 6, 14, 9, 16, 5, 18, 6, 16, 10, 19, 7, 19, 6, 17, 10, 18, 4, 21, 9, 17, 9, 19, 8
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OFFSET

0,6


COMMENTS

Related to Chen's theorem (Chen 1966, 1973) which states that every sufficiently large even number is the sum of a prime and another prime or semiprime. Yamada (2015) has proved that this holds for all even numbers larger than exp(exp(36)).
In terms of this sequence, Chen's theorem with Yamada's bound is equivalent to say that a(2*n) > 0 for all n > 1.7 * 10^1872344071119348 (exponent ~ 1.8*10^15).
Sequence A322007(n) = a(2n) lists the bissection corresponding to even numbers only.
A235645 lists the number of decompositions of 2n into a prime p and a prime or semiprime q; this is less than a(2n) because p + q and q + p is the same decomposition (if q is a prime), but this sequence will count the two distinct primes 2n  q and 2n  p (if q <> p).


REFERENCES

Chen, J. R. (1966). "On the representation of a large even integer as the sum of a prime and the product of at most two primes". Kexue Tongbao. 11 (9): 385386.
Chen, J. R. (1973). "On the representation of a larger even integer as the sum of a prime and the product of at most two primes". Sci. Sinica. 16: 157176.


LINKS

Table of n, a(n) for n=0..89.
Y. C. Cai, Chen's Theorem with Small Primes, Acta Mathematica Sinica 18, no. 3 (2002), pp. 597604. doi:10.1007/s101140200168.
P. M. Ross, On Chen's theorem that each large even number has the form (p1+p2) or (p1+p2p3), J. London Math. Soc. Series 2 vol. 10, no. 4 (1975), pp. 500506. doi:10.1112/jlms/s210.4.500.
Tomohiro Yamada, Explicit Chen's theorem, preprint arXiv:1511.03409 [math.NT] (2015).


EXAMPLE

a(4) = 1 is the first nonzero term corresponding to 4 = 2 + 2 or, rather, to the prime 2 = 4  2.
a(5) = 2 because the primes 2 = 5  3 and 3 = 5  2 are of the required form n  q where q = 3 resp. q = 2 are primes.
a(6) = 2 because the primes 2 = 6  4 and 3 = 6  3 are of the required form n  q, since q = 4 is a semiprime and q = 3 is a prime.


PROG

(PARI) A322006(n, s=0)=forprime(p=2, n2, bigomega(np)<3&&s++); s}


CROSSREFS

Cf. A322007, A235645, A045917, A130588, A241539.
Sequence in context: A236347 A045430 A067693 * A173419 A099053 A230697
Adjacent sequences: A322003 A322004 A322005 * A322007 A322008 A322009


KEYWORD

nonn


AUTHOR

M. F. Hasler, Jan 06 2019


STATUS

approved



