OFFSET
0,2
COMMENTS
It appears that 2, 3, 4, 6 are the only numbers k such that 2k can be expressed as the sum of two primes in only one way.
Except when n = 1, a(n) = A258713(n). The first 11 terms of this sequence are the same as the initial terms of A053033. If a(n) exists for all n then A053033 is a subsequence. - Andrew Howroyd, Jan 28 2020
LINKS
David A. Corneth, Table of n, a(n) for n = 0..16805 (first 1001 terms from Andrew Howroyd)
FORMULA
EXAMPLE
a(3) = 11: 22 = 3 + 19 = 5 + 17 = 11 + 11. Also 22 is the least number which could be expressed as the sum of two prime numbers in exactly three ways.
PROG
(PARI) a(n, lim=oo)={for(i=1, lim, my(s=0); forprime(p=2, i, s+=isprime(2*i-p)); if(s==n, return(i))); -1} \\ Andrew Howroyd, Jan 28 2020
CROSSREFS
KEYWORD
nonn
AUTHOR
K. B. Subramaniam (shunya_1950(AT)yahoo.co.in), Dec 24 2007
EXTENSIONS
a(0)=1 prepended, a(5) corrected and a(7) and beyond from Andrew Howroyd, Jan 28 2020
STATUS
approved