A325255 3-smooth numbers k such that k+1 and (k+2)/2 are prime.

2, 4, 12, 36, 72, 192, 1152, 2592, 2916, 1492992, 1990656, 5308416, 28311552, 6879707136, 1761205026816, 5566277615616, 79164837199872, 3799912185593856, 115422332637413376, 1332669751402954752, 4803028329503971872

Offset: 1

Author

Robert Israel, Sep 05 2019

Keywords

• nonn

Comments

Numbers k of the form 2^a*3^b such that k+1 and (k+2)/2 are prime.

All terms except 2 and 4 are in A325204.

All terms except 4 end in 2 or 6.

Examples

			a(3)=12 is a term because 12=2^2*3 and 13 and 14/2 are prime.
		

Links

• Ray Chandler, Table of n, a(n) for n = 1..41 (terms < 10^1000; first 39 terms from Robert Israel)

Crossrefs

Cf. A003586, A325204, A327240.

Programs

Maple

N:= 10^100: # to get terms <= N
sort(select(t -> isprime(t+1) and isprime((t+2)/2), [seq(seq(2^i*3^j, i = 1 .. ilog2(N/3^j)), j=0..floor(log[3](N)))]));