OFFSET
0,1
COMMENTS
From Daniel Suteu, Dec 28 2022: (Start)
For n >= 1, a(n) has the form k * 10^n + x, for some k >= 0, where x is a solution to the modular quadratic equation x^2 + x + d == 0 (mod 10^n), where d = q-p.
a(17) <= 379430283012423635659, a(18) <= 1857717470295105527413. (End)
EXAMPLE
a(2) = 2243 because 2243 is prime, the next prime is 2251, 2243^2+2251 = 5033300 = 10^2*50333 and 50333 is prime.
MAPLE
V:= Array(0..5):
count:= 0:
q:= 2:
while count < 6 do
p:= q; q:= nextprime(p);
v:= p^2+q;
r:= padic:-ordp(v, 2);
if r <= 5 and V[r] = 0 and padic:-ordp(v, 5) = r and isprime(v/10^r) then
V[r]:= p; count:= count+1;
fi;
od:
convert(V, list);
PROG
(PARI)
isok(n, p, q) = my(v=valuation(p^2+q, 10)); (v == n) && isprime((p^2+q)/10^v);
a(n) = my(p=2); forprime(q=p+1, oo, if(isok(n, p, q), return(p)); p=q); \\ Daniel Suteu, Apr 07 2022
CROSSREFS
KEYWORD
nonn,more
AUTHOR
J. M. Bergot and Robert Israel, Apr 05 2022
EXTENSIONS
a(6)-a(9) from Daniel Suteu, Apr 07 2022
a(10)-a(16) from Daniel Suteu, Dec 28 2022
STATUS
approved