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A259897
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a(n) is the 2-adic valuation of phi(binomial(2*n,n)).
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2
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0, 0, 1, 3, 3, 3, 4, 6, 6, 10, 9, 11, 10, 11, 11, 14, 12, 10, 11, 13, 14, 17, 17, 17, 16, 16, 18, 20, 22, 23, 21, 23, 20, 24, 21, 21, 20, 22, 24, 23, 22, 21, 22, 26, 26, 30, 31, 32, 30, 35, 34, 33, 31, 33, 34, 34, 33, 37, 38, 40, 42, 42, 44, 46, 42, 42, 43, 45
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OFFSET
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0,4
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COMMENTS
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The 2-adic valuation (or 2-adic order) of n>=1 is the highest exponent k such that 2^k divides n. Below we also write 2^k||n.
Since the number of primes in the interval (n, 2*n) tends to infinity as n goes to infinity, a(n) also tends to infinity (see formula).
By Kummer's theorem (link [Wikipedia]), a prime p divides binomial(2*n,n) if and only if there is a carry in adding n+n in base p.
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LINKS
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FORMULA
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c(n)-1 + Sum_{n < prime p < 2*n)c_p <= a(n) <= Sum_{2 < prime p < 2 *n)c_p, n>1, where 2^c(n)|| binomial(2*n,n) (note that c(n) = A000120(n)) and 2^c_p || p-1.
a(n) = (number of carries in binary addition of n+n) - 1 + Sum(p in S, A007814(p-1)) where S is the set of odd primes p < 2n such that at least one of the base-p digits of n is greater than (p-1)/2. - Robert Israel, Jul 10 2015
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EXAMPLE
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Let n=9, binomial(18,9) = 48620. Here
c(n)=2, Sum_{9 < prime p < 18)c_p = 7,
Sum_{2 < prime p < 18)c_p = 11. So 8<=a(9) <= 11.
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MAPLE
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seq(padic:-ordp(numtheory:-phi(binomial(2*n, n)), 2), n= 0 .. 100); # Robert Israel, Jul 10 2015
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MATHEMATICA
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Map[IntegerExponent[EulerPhi[Binomial[2#, #]], 2]&, Range[0, 100]]
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PROG
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(PARI) vector(80, n, n--; valuation(eulerphi(binomial(2*n, n)), 2)) \\ Michel Marcus, Jul 11 2015
(Python)
from math import comb
from sympy import totient
def A259897(n): return (~(m:=totient(comb(2*n, n)))& m-1).bit_length() # Chai Wah Wu, Jul 07 2022
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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