OFFSET
1,1
COMMENTS
The resulting primes are the generalized cuban primes of the form (x^3-y^3)/(x-y), x=y+2 (see A002648). - Jani Melik, Jul 18 2007
LINKS
Zak Seidov, Table of n, a(n) for n = 1..1000
FORMULA
a(n) = sqrt((A002648(n)-1)/3). - Zak Seidov, Feb 04 2016
EXAMPLE
1 + 3*2^2 = 13 = A002648(1) is the 1st prime of this form, so a(1) = 2.
1 + 3*6^2 = 109 = A002648(2) is the 2nd prime of this form, so a(2) = 6.
1 + 3*8^2 = 193 = A002648(3) is the 3rd prime of this form, so a(3) = 8.
If m=98 then 3*m^2 + 1 = 28813 = A002648(19) is prime (the 19th of this form), so 98 is a term (the 19th).
MAPLE
ts_kubpra_ind:=proc(n) local i, tren, ans; ans:=[ ]: for i from 0 to n do tren:=1+3*i^2: if (isprime(tren)='true') then ans:=[ op(ans), i ] fi od: RETURN(ans); end: ts_kubpra_ind(2000); # Jani Melik, Jul 18 2007
MATHEMATICA
Select[Range[400], PrimeQ[3#^2+1]&] (* Harvey P. Dale, Jul 17 2016 *)
PROG
(PARI) is(n)=isprime(3*n^2+1) \\ Charles R Greathouse IV, Feb 07 2017
CROSSREFS
KEYWORD
nonn
AUTHOR
Parthasarathy Nambi, Oct 06 2005
EXTENSIONS
More terms from Jani Melik, Jul 18 2007
Edited by N. J. A. Sloane, Sep 28 2007
STATUS
approved