A381974 Primes of the form Sum_{k >= 0} floor(m/3^k) for some number m.

2, 5, 13, 17, 19, 23, 31, 41, 53, 59, 61, 67, 71, 89, 97, 101, 103, 107, 127, 131, 139, 149, 151, 157, 163, 167, 179, 191, 193, 197, 211, 223, 227, 229, 233, 251, 257, 263, 269, 277, 283, 313, 317, 331, 337, 349, 353, 373, 379, 383, 409, 419, 421, 431, 439

Offset: 1

Author

Clark Kimberling, Apr 01 2025

Keywords

• nonn

Examples

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

Links

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

Crossrefs

Cf. A000040, A381973. Includes A076481.

Programs

Maple

f:= proc(n) local k; add(floor(n/3^k),k=0..ilog[3](n)) end proc:
select(isprime, map(f, [$2..100])); # 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 *)