OFFSET
1,1
COMMENTS
The above equation is one of a family of twin prime average-generating quadratics y = x^2 + x - c, where c can be any even integer not of the form 6d + 4.
For f(x) = x^2 + x - c, f(-x) = f(x-1).
If c = 0, the positive x that satisfy y = x^2 + x - c are A088485.
LINKS
Michael G. Kaarhus and Charles R Greathouse IV, Table of n, a(n) for n = 1..10000 (first 334 terms from Kaarhus)
M. G. Kaarhus, A Family of Twin Prime Quads (PDF)
EXAMPLE
x = 2, x = 4, x = 10, x = 13, x = 16
x = 28, x = 64, x = 70, x = 76, x = 85
MATHEMATICA
s = {4}; Do[If[PrimeQ[n - 1] && PrimeQ[n + 1] && IntegerQ[Sqrt[9 + 4 n]], AppendTo[s, n]], {n, 18, 453600, 6}]; s (* Zak Seidov, Mar 21 2013 *)
Select[Mean/@Select[Partition[Prime[Range[100000]], 2, 1], #[[2]]-#[[1]]==2&], IntegerQ[ Sqrt[ 9+4#]]&] (* Harvey P. Dale, Aug 18 2024 *)
PROG
(PARI) p=2; forprime(q=3, 1e6, if(q-p>2, p=q; next); n=sqrtint(y=(p+q)\2); if(n^2+n-2==y, print1(y", ")); p=q) \\ Charles R Greathouse IV, Mar 20 2013
(PARI) test(y)=if(isprime(y-1)&&isprime(y+1), print1(", "y))
print1(4); for(n=0, 100, test(18*n*(2*n+1)); test(18*(2*n^2+3*n+1))) \\ Charles R Greathouse IV, Mar 20 2013
CROSSREFS
KEYWORD
nonn
AUTHOR
Michael G. Kaarhus, Mar 07 2013
STATUS
approved