A381973 Numbers m such that Sum_{k >= 0} floor(m/3^k) is prime.

2, 4, 9, 12, 14, 17, 22, 28, 36, 41, 42, 46, 49, 61, 66, 69, 71, 73, 86, 89, 94, 101, 102, 107, 110, 113, 121, 129, 131, 134, 143, 151, 153, 155, 158, 169, 173, 177, 181, 187, 190, 211, 214, 223, 227, 235, 238, 250, 254, 257, 274, 281, 282, 289, 295, 301

Offset: 1

Author

Clark Kimberling, Apr 01 2025

Keywords

• nonn

Comments

Includes 3^(k-1) for k in A028491. - Robert Israel, Apr 21 2025

Examples

			[9/1] + [9/3] + [9/9] = 13, where [ ] = floor, so 9 is in the sequence.
		

Links

• Robert Israel, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A000040, A028491, A381974.

Programs

Maple

f:= proc(n) add(floor(n/3^k),k=0..ilog[3](n)) end proc:
select(m -> isprime(f(n)), [$2..1000]); # Robert Israel, Apr 21 2025

Mathematica

f[n_] := Sum[Floor[n/3^k], {k, 0, Floor[Log[3, n]]}]  (* A004128 *)
u = Select[Range[400], PrimeQ[f[#]] &]  (* A381973 *)
Map[f, u]   (* A381974 *)