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A338989
a(n) is the first prime p such that the sum of 2*n consecutive primes starting at p is q*(q+1) where q is prime, or 0 if there is no such p.
2
5, 71, 3, 977, 37, 7829, 8681, 283, 14341, 37181, 31, 8839, 1181, 60901, 54727, 2579, 64901, 1248019, 43, 141803, 47, 29881, 991, 5, 881, 1603919, 31123, 18679, 174149, 74149, 11, 1328269, 925513, 447859, 61, 890969, 5867, 35759, 4093, 27239, 1549, 6551, 1901987, 4597, 64781, 307, 13121, 353
OFFSET
1,1
LINKS
EXAMPLE
a(3) = 3 because the sum of the 2*3=6 consecutive primes starting at 3 is 3+5+7+11+13+17 = 56 = 7*(7+1) where 7 is prime.
a(4) = 977 because the sum of the 2*4=8 consecutive primes starting at 977 is 977+983+991+997+1009+1013+1019+1021 = 8010 = 89*(89+1) where 89 is prime.
MAPLE
N:= 10^5:
P:= select(isprime, [2, seq(i, i=3..N, 2)]):
S:= ListTools:-PartialSums([0, op(P)]):
nP:= nops(S):
f:= proc(n) local i;
for i from 1 to nP-n do
if issqr(1+4*(S[i+n]-S[i])) and isprime((sqrt(1+4*(S[i+n]-S[i]))-1)/2)then return P[i] fi
od;
FAIL
end proc:
R:= NULL:
for i from 1 do
v:= f(2*i);
if v = FAIL then break fi;
R:= R, v
od:
R;
CROSSREFS
Sequence in context: A015502 A303291 A324229 * A101019 A056266 A248367
KEYWORD
nonn
AUTHOR
J. M. Bergot and Robert Israel, Dec 20 2020
STATUS
approved