OFFSET
1,1
COMMENTS
Numbers n such that n+25, 2n+125, and 3n+625 are prime.
A geometric-arithmetic progression of primes is a set of k primes (denoted by GAP-k) of the form p r^j + j d for fixed p, r and d and consecutive j. Symbolically, for r = 1, this sequence simplifies to the familiar primes in arithmetic progression (denoted by AP-k). The computations were done without any assumptions on the form of d. Primality requires d to be even and coprime to 5.
This sequence is infinite on Dickson's conjecture. [Charles R Greathouse IV, Mar 12 2012]
LINKS
Sameen Ahmed Khan, Table of n, a(n) for n = 1..10000
Sameen Ahmed Khan, Primes in Geometric-Arithmetic Progression, arXiv:1203.2083v1 [math.NT], (Mar 09 2012).
EXAMPLE
d = 12 then {5*5^j + j*d}, j = 0 to 3, is {5, 37, 149, 661}, which is 4 primes in geometric-arithmetic progression.
MATHEMATICA
p = 5; gapset4d = {}; Do[If[PrimeQ[{p, p*p + d, p*p^2 + 2*d, p*p^3 + 3*d}] == {True, True, True, True}, AppendTo[gapset4d, d]], {d, 0, 1000, 2}]; gapset4d
Select[Range[2, 1100, 2], And@@PrimeQ[{#+25, 2#+125, 3#+625}]&] (* Harvey P. Dale, Jan 06 2013 *)
PROG
(PARI) forstep(n=2, 1e3, [2, 2, 2, 4], if(isprime(n+25)&&isprime(2*n+125)&&isprime(3*n+625), print1(n", "))) \\ Charles R Greathouse IV, Mar 12 2012
CROSSREFS
KEYWORD
nonn
AUTHOR
Sameen Ahmed Khan, Mar 06 2012
STATUS
approved