cp's OEIS Frontend

A306247 Numbers k such that 2k - p is not a prime where p is any prime divisor of 4k^2 - 1.

%I A306247 #25 Aug 09 2019 16:52:20
%S A306247 1,2,3,6,9,14,15,19,21,26,29,30,34,36,40,48,49,51,54,61,63,64,69,74,
%T A306247 75,79,82,84,86,89,90,95,96,99,103,106,110,111,112,114,119,120,135,
%U A306247 139,141,146,147,149,151,152,153,154,156,161,166,169,173,174,179,180,184,186,187,190,194
%N A306247 Numbers k such that 2k - p is not a prime where p is any prime divisor of 4k^2 - 1.
%C A306247 Primes in a(n): 2, 3, 19, 29, 61, 79, 89, 103, 139, 149, 151, 173, 179, ...
%F A306247 A306261(a(n)) > 1 for n >= 4.
%e A306247 1 is a term because 4*1^2 - 1 = 3 and 2*1 - 3 = -1 (nonprime);
%e A306247 2 is a term because 4*2^2 - 1 = 15 and 2*2 - 15 = -11 (nonprime);
%e A306247 3 is a term because 4*3^2 - 1 = 35 and 2*3 - 35 = -29 (nonprime);
%e A306247 6 is a term because 4*6^2 - 1 = 143 = 11*13 and 2*6 - 11 = 1 (nonprime), 2*6 - 13 = -1 (nonprime);
%e A306247 9 is a term because 4*9^2 - 1 = 323 = 17*19 and 2*9 - 17 = 1 (nonprime), 2*9 - 19 = -1 (nonprime).
%p A306247 filter:= proc(n) andmap(`not` @ isprime, map(p -> 2*n-p, numtheory:-factorset(4*n^2-1))) end proc:
%p A306247 select(filter, [$1..300]); # _Robert Israel_, Jan 31 2019
%t A306247 Select[Range@ 200, AllTrue[2 # - FactorInteger[4 #^2 - 1][[All, 1]], ! PrimeQ@ # &] &] (* _Michael De Vlieger_, Feb 03 2019 *)
%o A306247 (PARI) isok(k) = {my(pf = factor(4*k^2-1)[,1]); #select(x->isprime(2*k-x), pf) == 0;} \\ _Michel Marcus_, Mar 02 2019
%Y A306247 Includes A040040.
%Y A306247 Cf. A306261.
%K A306247 nonn,easy
%O A306247 1,2
%A A306247 _Juri-Stepan Gerasimov_, Jan 31 2019