A285804 - OEIS

A285804

Composite numbers of the form 12*k+5 or 12*k+7 for some k.

55, 65, 77, 91, 115, 125, 161, 175, 185, 187, 209, 221, 235, 245, 247, 259, 295, 305, 319, 329, 341, 343, 355, 365, 377, 391, 403, 413, 415, 425, 427, 437, 451, 473, 475, 485, 497, 511, 533, 535, 545, 559, 581, 583, 595, 605, 629, 655, 665

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,1

COMMENTS

Conjecture: Let r(n) = (a(n)-1)/(a(n)+1)) if a(n) mod 4 = 1, (a(n)+1)/(a(n)-1)) otherwise; then Product_{n>=1} r(n) = (28/27) * (32/33) * (38/39) * (46/45) * ... = 1.

LINKS

Table of n, a(n) for n=1..49.

MAPLE

remove(isprime, [seq(seq(12*k+j, j=[5, 7]), k=1..100)]); # Robert Israel, Apr 28 2017

PROG

(PARI) {

for(n=0, 100,

n12=12*n; n5=n12+5; n7=n12+7;

if(!isprime(n5), print1(n5", "));

if(!isprime(n7), print1(n7", "))

)

}