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A023520
Exponent of 2 in prime factorization of prime(n)*prime(n-1) - 1.
3
0, 1, 1, 2, 1, 2, 1, 2, 1, 1, 1, 2, 1, 2, 1, 1, 1, 1, 2, 1, 1, 2, 1, 3, 2, 1, 2, 1, 2, 1, 2, 1, 1, 1, 1, 1, 1, 2, 1, 1, 1, 1, 1, 2, 1, 2, 2, 2, 1, 2, 1, 1, 1, 1, 1, 1, 1, 1, 2, 1, 1, 1, 2, 1, 2, 1, 1, 1, 1, 2, 1, 3, 1, 1, 2, 1, 6, 2, 3
OFFSET
2,4
COMMENTS
a(n) = 1 if and only if A080378(n-1) = 2. - Robert Israel, Feb 07 2018
LINKS
FORMULA
a(n) = A007814(A023515(n)). - Michel Marcus, Sep 30 2013
MAPLE
seq(padic:-ordp(ithprime(n)*ithprime(n-1)-1, 2), n=2..200); # Robert Israel, Feb 07 2018
PROG
(PARI) a(n) = valuation(prime(n)*prime(n-1) - 1, 2); \\ Michel Marcus, Sep 30 2013
(Python)
from sympy import prime
def A023520(n): return (~(m:=prime(n)*prime(n-1)-1)& m-1).bit_length() # Chai Wah Wu, Jul 07 2022
CROSSREFS
KEYWORD
nonn
EXTENSIONS
Offset set to 2 and a(2) corrected by Michel Marcus, Sep 30 2013
STATUS
approved