

A127079


Number of ways to represent prime(n) as a+b with a >= b > 0 and a^2+b^2 prime.


1



1, 1, 2, 2, 3, 3, 4, 4, 4, 6, 5, 8, 9, 7, 7, 9, 9, 11, 11, 9, 11, 13, 15, 14, 14, 18, 16, 17, 16, 20, 18, 22, 18, 21, 23, 21, 24, 24, 22, 24, 22, 28, 30, 23, 27, 24, 29, 30, 30, 28, 29, 24, 28, 30, 34, 33, 36, 35, 31, 37, 32, 36, 37, 41, 42, 42, 42, 43, 42, 38, 34, 43, 38, 45, 44
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OFFSET

1,3


COMMENTS

Essentially A036468 restricted to the primes.
a(n) <= floor(prime(n)/2).


LINKS



EXAMPLE

prime(5) = 11 can be represented as 10+1, 9+2, 8+3, 7+4 and 6+5. Among 10^2+1^2 = 101, 9^2+2^2 = 85, 8^2+3^2 = 73, 7^2+4^2 = 65 and 6^2+5^2 = 61 are three primes, hence a(5) = 3.


MAPLE

f:= proc(n) local a;
add(charfcn[{true}](isprime(a^2 + (na)^2)), a=1..n/2)
end proc:
map(f, [seq(ithprime(i), i=1..100)]); # Robert Israel, Jun 03 2019


MATHEMATICA

Reap[Do[p = Prime[n]; c = 0; Do[b = p  a; If[PrimeQ[a^2 + b^2], c++], {a, 1, p/2}]; Sow[c], {n, 1, 75}]][[2, 1]] (* JeanFrançois Alcover, Aug 19 2020 *)


PROG

(PARI) {for(n=1, 75, p=prime(n); c=0; for(a=1, p\2, b=pa; if(isprime(a^2+b^2), c++)); print1(c, ", "))} /* Klaus Brockhaus, Mar 26 2007 */


CROSSREFS



KEYWORD

nonn


AUTHOR



EXTENSIONS



STATUS

approved



