A053177 - OEIS

%I #24 Apr 01 2019 14:10:23

%S 15,27,35,39,63,75,87,95,119,123,135,143,147,159,195,203,207,215,219,

%T 255,275,279,299,303,315,327,335,363,387,395,399,423,447,455,459,483,

%U 515,527,539,543,555,567,615,623,627,635,663,675,695,699,707,735,747

%N Odd composite k such that (k-1)/2 is prime.

%C Composite numbers produced in A053176.

%H Michael De Vlieger, <a href="/A053177/b053177.txt">Table of n, a(n) for n = 1..10000</a>

%H R. P. Boas & N. J. A. Sloane, <a href="/A005381/a005381.pdf">Correspondence, 1974</a>

%F From the composite, subtract 1, divide by 2 and result is a prime.

%e a(3)=35 and 35-1=34, 34/2=17, prime.

%t Select[2 Prime@ Range@ 74 + 1, CompositeQ] (* _Michael De Vlieger_, Jul 13 2015 *)

%t Select[Range[1,801,2],CompositeQ[#]&&PrimeQ[(#-1)/2]&] (* _Harvey P. Dale_, Apr 01 2019 *)

%o (PARI) main(size)={my(v=vector(size),i,t=1);for(i=1, size, while(isprime(2*prime(t)+1), t++); v[i]=2*prime(t)+1;t++;);return(v)} /* _Anders Hellström_, Jul 13 2015 */

%Y Cf. A053176, A005385, A005382, A259978.

%K easy,nonn

%O 1,1

%A _Enoch Haga_, Feb 29 2000

%E Definition clarified by _Peter Munn_, Oct 26 2017