OFFSET
1,1
COMMENTS
Let c^2 + d^2 = p be a prime, A002313(n). Then x^2 + y^2 = q is the smallest prime, a(n), such that cx + dy = 1 (Bézout's identity), where c,d,x,y are integers. We have pq = m^2 + 1 at m = cy - dx.
a(n) is the smallest prime q such that q*A002313(n)-1 is a square. - Thomas Ordowski, Sep 13 2015
Conjecture: a(n) < A002313(n)^2 for n > 1. - Thomas Ordowski, Dec 28 2017
LINKS
Robert Israel, Table of n, a(n) for n = 1..2910
EXAMPLE
For prime 2 = 1^2 + 1^2 is 1*2 + 1*(-1) = 1 and 2^2 + (-1)^2 = 5 is prime, so a(1) = 5. For A002313(2) = 5 is vice versa so a(2) = 2.
MAPLE
N:= 10^6: # to get all a(n) before the first one > N
P:= select(isprime, [2, seq(4*i+1, i=1..floor((N-1)/4))]):
f:= proc(p) local i;
for i from 1 to nops(P) do
if issqr(p*P[i]-1) then return P[i] fi
od:
-1
end proc:
for i from 1 to nops(P) do
v:= f(P[i]);
if v = -1 then break fi;
A[i]:= v;
od:
seq(A[j], j=1..i-1); # Robert Israel, Sep 13 2015
PROG
(PARI)
\\ cs should contain terms from A002330
\\ ds should contain terms from A002331
a244290(cs, ds) = {
vector(#cs, i,
c=cs[i]; d=ds[i]; [u, v]=gcdext(c, d);
x=u; y=v; while(!isprime(x^2+y^2), x+=d; y-=c); e=x^2+y^2;
x=u; y=v; while(!isprime(x^2+y^2), x-=d; y+=c); f=x^2+y^2;
min(e, f)
)
} \\ Colin Barker, Jul 06 2014
CROSSREFS
KEYWORD
nonn
AUTHOR
Thomas Ordowski, Jun 27 2014
EXTENSIONS
More terms from Colin Barker, Jul 06 2014
STATUS
approved