A130097 - OEIS

%I #2 Mar 31 2012 14:40:00

%S 3,5,11,13,17,23,29,31,41,43,47,53,59,73,79,83,89,101,103,109,113,131,

%T 137,139,149,151,163,167,173,179,181,191,193,197,199,211,223,227,233,

%U 239,241,251,257,263,269,271,277,281,283,293,307,311,313,317,331,337

%N Primes prime(n) such that at least one of the two numbers (prime(n+2)^2-prime(n)^2)/2 - 1 and (prime(n+2)^2-prime(n)^2)/2 + 1 is not prime.

%e a(1)=3 because (7^2 - 3^2)/2 - 1 = 19 and (7^2 - 3^2)/2 + 1 = 21 (21 is not prime),

%e a(2)=5 because (11^2 - 5^2)/2 - 1 = 47 and (11^2 - 5^2)/2 + 1 = 49 (49 is not prime),

%e a(3)=11 because (17^2 - 11^2)/2 - 1 = 83 and (17^2 - 11^2)/2 + 1 = 85 (85 is not prime), ...

%p ts_p2_20:=proc(n) local a,b,i,ans; ans := [ ]: for i from 2 to n do a := (ithprime(i+2)^(2)-ithprime(i)^(2))/2-1: b := (ithprime(i+2)^(2)-ithprime(i)^(2))/2+1: if not (isprime(a)=true and isprime(b)=true) then ans := [ op(ans), ithprime(i) ]: fi od; RETURN(ans) end: ts_p2_20(300);

%Y Cf. A130761.

%K nonn

%O 1,1

%A _Jani Melik_, Aug 01 2007