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A178659
Numbers n such that n^2 +- (n-1)^2 are primes.
3
2, 3, 6, 10, 15, 30, 31, 36, 40, 51, 66, 70, 91, 100, 136, 175, 190, 205, 225, 231, 261, 285, 286, 316, 321, 331, 370, 376, 411, 441, 465, 496, 516, 520, 526, 535, 546, 565, 576, 586, 591, 681, 720, 730, 745, 750, 766, 855, 871, 906, 916, 951, 975, 1081, 1120
OFFSET
1,1
LINKS
FORMULA
a(n) = 1 + A068501(n). - Zak Seidov, Feb 10 2015
EXAMPLE
2 is in the sequence because 2^2 + 1^2 = 5 and 2^2 - 1^2 = 3 are both prime.
3 is in the sequence because 3^2 + 2^2 = 13 and 3^2 - 2^2 = 5 are both prime.
MATHEMATICA
lst={}; Do[If[PrimeQ[n^2-(n-1)^2]&&PrimeQ[n^2+(n-1)^2], AppendTo[lst, n]], {n, 7!}]; lst
Select[Range[8!], PrimeQ[#^2 -(#-1)^2] && PrimeQ[#^2 +(#-1)^2] &] (* G. C. Greubel, Jan 28 2019 *)
PROG
(PARI) A178659()={my(maxx=1000); n=2; ptr=0;
while(n<=maxx, q1=n^2-(n-1)^2; q2=n^2+(n-1)^2;
if(isprime(q1)&&isprime(q2), ptr++; write("b178659.txt", ptr, " ", n)); n++); } \\ Bill McEachen, Jun 13 2014
CROSSREFS
Cf. A068501.
Sequence in context: A280421 A369425 A018141 * A268064 A077011 A246868
KEYWORD
nonn
AUTHOR
STATUS
approved