The On-Line Encyclopedia of Integer Sequences (OEIS)

Revision History for A005090 (Bold, blue-underlined text is an addition; faded, red-underlined text is a ~~deletion~~.)
Showing entries 1-10 | older changes

A005090

Number of primes == 2 mod 3 dividing n.
(history; published version)

#28 by N. J. A. Sloane at Wed Apr 14 22:23:48 EDT 2021

STATUS

editing

approved

#27 by N. J. A. Sloane at Wed Apr 14 22:23:45 EDT 2021

NAME

Number of primes = = 2 mod 3 dividing n.

STATUS

approved

editing

#26 by N. J. A. Sloane at Wed Apr 14 22:23:34 EDT 2021

FORMULA

a(n) = (1/2)*Sum_{p|n} (p*(p - 1) mod 3), where p is a prime. - Ridouane Oudra, Mar 21 2021

STATUS

proposed

approved

#25 by Ridouane Oudra at Sun Mar 21 08:33:21 EDT 2021

STATUS

editing

proposed

Discussion

Sun Mar 21

13:08

Joerg Arndt: (p*(p - 1) mod 3) looks like an obfuscation of "p == 2 (mod 3)"

Sun Mar 28

14:45

Antti Karttunen: Joerg: the intent is clearly different there. See (1/2)* in the front of the Sum.

Tue Apr 13

09:38

Andrey Zabolotskiy: I agree that this looks like an obfuscation to the name. Suggest reverting, unless this formula links this sequence to some other notable sequences that are produced by similar formulas and such generalization cannot be expressed in a more clear manner.

Wed Apr 14

22:23

N. J. A. Sloane: will revert

#24 by Ridouane Oudra at Sun Mar 21 08:32:24 EDT 2021

FORMULA

a(n) = (1/2)*Sum_{p|n} (p*(p - 1) mod 3), where p is a prime. - Ridouane Oudra, Mar 21 2021

STATUS

approved

editing

#23 by Alois P. Heinz at Sat May 02 15:41:33 EDT 2020

STATUS

proposed

approved

#22 by F. Chapoton at Sat May 02 13:26:35 EDT 2020

STATUS

editing

proposed

#21 by F. Chapoton at Sat May 02 13:26:24 EDT 2020

PROG

def a(n): return sum([1 for p in primefactors(n) if p%3==2])

print map([a, (n) for n in range(1, 101)]) # Indranil Ghosh, Jul 11 2017

STATUS

approved

editing

Discussion

Sat May 02

13:26

F. Chapoton: adapt code to  python3

#20 by N. J. A. Sloane at Sat Dec 07 12:33:53 EST 2019

PROG

print map(a, xrangerange(1, 101)) # Indranil Ghosh, Jul 11 2017

Discussion

Sat Dec 07

12:33

OEIS Server: https://oeis.org/edit/global/2838

#19 by N. J. A. Sloane at Thu Jul 13 21:09:48 EDT 2017

STATUS

proposed

approved