OFFSET
1,4
COMMENTS
a(n) = A049847(n) for n = 1..46. The values of n such that a(n) is different from A049847(n) are 47, 59, 62, 72, 93, 102, 108, 123, 144, 149, 152, 161, 164, 171, 182, 197, 203, 207, 213, 227, ...
The corresponding pairs of primes are (3, 3), (3, 5), (3, 7), (5, 7), (3, 11), (3, 13), (5, 13), (3, 17), ... (A210957).
LINKS
Michel Lagneau, Table of n, a(n) for n = 1..10000
Jon Maiga, Computer-generated formulas for A255274, Sequence Machine.
FORMULA
a(n) = n + (3-A020482(n+2))/2 = (A020481(n+2)-1)/2. - Bill McEachen, Jan 02 2022 [Should these formulas be credited to Jon Maiga? - N. J. A. Sloane, Feb 06 2022]
EXAMPLE
a(13)=3 because 2*13 + 4 = 30 => 13 pairs (27,3), (25,5), (23,7), ..., (3,27) and the pair (23,7) is the third pair having prime elements.
MAPLE
nn:=100:for n from 6 by 2 to nn do:ii:=0:it:=1:for p from 3 by 2 to n while(ii=0) do:if type(n-p, prime)=true and type(p, prime)=true then ii:=1: printf(`%d, `, it):else it:=it+1:fi:od:od:
PROG
(PARI) a(n)=my(m=2*n+4); forprime(q=3, n+2, if(isprime(m-q), return(q\2))) \\ Charles R Greathouse IV, Jan 07 2022
CROSSREFS
KEYWORD
nonn
AUTHOR
Michel Lagneau, Feb 20 2015
EXTENSIONS
Edited by N. J. A. Sloane, Sep 12 2017
STATUS
approved