OFFSET
1,7
COMMENTS
We use a little-known conjecture by Goldbach on the primes of form n^2+1: let A be the set of all numbers a for which a^2+1 is prime (A={1, 2, 4, 6, 10, ...}). Then every a in A (a>1) can be written in the form a=b+c for b,c in A.
LINKS
Mathoverflow, Primes of the form a^2+1
EXAMPLE
a(20)=5 because A005574(20)= 110 =>
MAPLE
T:=array(1..112):
nn:=1000:k:=0:
for i from 1 to nn do:
p:=i^2+1:if type(p, prime)=true
then
k:=k+1:T[k]:=i:
else fi:
od:
for n from 1 to k do:q:=T[n]:it:=0:
for a from 1 to k do:p1:=T[a]:
for b from a to k do:p2:=T[b]:
if q=p1+p2
then
it:=it+1:
else fi:
od:
od:
printf(`%d, `, it):
od:
CROSSREFS
KEYWORD
nonn
AUTHOR
Michel Lagneau, Jul 09 2015
STATUS
approved