The On-Line Encyclopedia of Integer Sequences (OEIS)

Revision History for A166684 (Bold, blue-underlined text is an addition; faded, red-underlined text is a ~~deletion~~.)
Showing entries 1-10 | older changes

A166684

Numbers n such that d(n)<4.
(history; published version)

#28 by N. J. A. Sloane at Fri Aug 09 15:15:24 EDT 2024

STATUS

reviewed

approved

#27 by Michel Marcus at Fri Aug 09 13:34:26 EDT 2024

STATUS

proposed

reviewed

#26 by Chai Wah Wu at Fri Aug 09 13:18:24 EDT 2024

STATUS

editing

proposed

#25 by Chai Wah Wu at Fri Aug 09 13:18:02 EDT 2024

PROG

return int(m) # Chai Wah Wu, Aug 09 2024

#24 by Chai Wah Wu at Fri Aug 09 13:17:28 EDT 2024

PROG

(Python)

from math import isqrt

from sympy import primepi

def A166684(n):

def f(x): return n-1+x-primepi(x)-primepi(isqrt(x))

m, k = n, f(n)

while m != k:

m, k = k, f(k)

return int(m) # Chai Wah Wu, Aug 09 2024

STATUS

approved

editing

#23 by Michael De Vlieger at Fri May 26 22:33:10 EDT 2023

STATUS

proposed

approved

#22 by Peter Munn at Fri May 26 19:03:39 EDT 2023

STATUS

editing

proposed

#21 by Peter Munn at Fri May 26 19:01:44 EDT 2023

COMMENTS

Numbers that cannot be written as a product of 2 distinct nonunits. - Peter Munn, May 26 2023

CROSSREFS

Cf. A000005, A229964.

STATUS

approved

editing

#20 by Charles R Greathouse IV at Fri Dec 23 09:58:44 EST 2022

STATUS

editing

approved

#19 by Charles R Greathouse IV at Fri Dec 23 09:58:42 EST 2022

PROG

(PARI) is(n)=isprime(n) || (issquare(n, &n) && isprime(n)) || n==1 \\ Charles R Greathouse IV, Dec 23 2022

CROSSREFS

A000430 is the main entry for this sequence.

Cf. A000005, A000430.

KEYWORD

nonn,easy

STATUS

approved

editing