A210994 Numbers k such that A000005(k) != 4.

1, 2, 3, 4, 5, 7, 9, 11, 12, 13, 16, 17, 18, 19, 20, 23, 24, 25, 28, 29, 30, 31, 32, 36, 37, 40, 41, 42, 43, 44, 45, 47, 48, 49, 50, 52, 53, 54, 56, 59, 60, 61, 63, 64, 66, 67, 68, 70, 71, 72, 73, 75, 76, 78, 79, 80, 81, 83, 84, 88, 89, 90, 92, 96

Offset

1,2

Links

Table of n, a(n) for n=1..64.

Example

6 is not in the sequence because 6 has four divisors: 1, 2, 3, 6.

Mathematica

Select[Range[100], DivisorSigma[0, #]!=4&] (* Harvey P. Dale, Jan 07 2013 *)

Prog

(Python)
from math import isqrt
from sympy import primepi, integer_nthroot, primerange
def A210994(n):
    def f(x): return int(n+primepi(integer_nthroot(x, 3)[0])-(t:=primepi(s:=isqrt(x)))-(t*(t-1)>>1)+sum(primepi(x//k) for k in primerange(1, s+1)))
    m, k = n, f(n)
    while m != k: m, k = k, f(k)
    return m # Chai Wah Wu, Dec 01 2025

Crossrefs

Complement of A030513. Column 4 of A210976.

Cf. A018252, A213367.

Sequence in context: A395045 A117290 A286972 * A331593 A381097 A319237

Adjacent sequences: A210991 A210992 A210993 * A210995 A210996 A210997

Keyword

nonn

Author

Omar E. Pol, Aug 11 2012

Status

approved