A053177 Odd composite k such that (k-1)/2 is prime.

15, 27, 35, 39, 63, 75, 87, 95, 119, 123, 135, 143, 147, 159, 195, 203, 207, 215, 219, 255, 275, 279, 299, 303, 315, 327, 335, 363, 387, 395, 399, 423, 447, 455, 459, 483, 515, 527, 539, 543, 555, 567, 615, 623, 627, 635, 663, 675, 695, 699, 707, 735, 747

Offset

1,1

Comments

Composite numbers produced in A053176.

Links

Michael De Vlieger, Table of n, a(n) for n = 1..10000

R. P. Boas & N. J. A. Sloane, Correspondence, 1974

Formula

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

Example

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

Mathematica

Select[2 Prime@ Range@ 74 + 1, CompositeQ] (* Michael De Vlieger, Jul 13 2015 *)
Select[Range[1, 801, 2], CompositeQ[#]&&PrimeQ[(#-1)/2]&] (* Harvey P. Dale, Apr 01 2019 *)

Prog

(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 */

Crossrefs

Cf. A053176, A005385, A005382, A259978.

Sequence in context: A091236 A354714 A307854 * A349175 A145195 A096022

Adjacent sequences: A053174 A053175 A053176 * A053178 A053179 A053180

Keyword

easy,nonn

Author

Enoch Haga, Feb 29 2000

Extensions

Definition clarified by Peter Munn, Oct 26 2017

Status

approved