A240668 - OEIS

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A240668

Number of the first odd exponents in the prime power factorization of (2*n)!.

1, 2, 0, 1, 0, 0, 2, 1, 0, 0, 2, 0, 1, 2, 0, 1, 0, 0, 2, 0, 3, 3, 0, 0, 1, 2, 0, 1, 0, 0, 1, 1, 0, 0, 1, 0, 1, 2, 0, 0, 1, 5, 0, 1, 0, 0, 3, 0, 1, 1, 0, 2, 0, 0, 2, 1, 0, 0, 3, 0, 1, 2, 0, 3, 0, 0, 2, 0, 5, 2, 0, 0, 1, 3, 0, 1, 0, 0, 2, 0, 1, 1, 0, 1, 0, 0, 4

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OFFSET

1,2

COMMENTS

According to Chen's theorem, the sequence is unbounded.

LINKS

Peter J. C. Moses, Table of n, a(n) for n = 1..10000

D. Berend, Parity of exponents in the factorization of n!, J. Number Theory, 64 (1997), 13-19.

Y.-G. Chen, On the parity of exponents in the standard factorization of n!, J. Number Theory, 100 (2003), 326-331.

FORMULA

a(n)*A240606(n) = 0.

EXAMPLE

32! = 2^31*3^14*5^7*7^4*11^2*13^2*17*19*23*29*31, and only the first 1 exponent is odd, so a(16) = 1.

MATHEMATICA

Map[Count[First[Split[Mod[Last[Transpose[FactorInteger[(2*#)!]]], 2]]], 1]&, Range[75]] (* Peter J. C. Moses, Apr 10 2014 *)

PROG

(PARI) a(n) = {my(f = factor((2*n)!)); my(nb = 0); my(i = 1); while((i <= #f~) && (f[i, 2] % 2), nb++; i++; ); nb; } \\ Michel Marcus, Apr 10 2014

CROSSREFS

Cf. A240537, A240606, A240619, A240620.

Sequence in context: A377432 A104451 A285680 * A106602 A106594 A357070

Adjacent sequences: A240665 A240666 A240667 * A240669 A240670 A240671

KEYWORD

nonn

AUTHOR

Vladimir Shevelev, Apr 10 2014

EXTENSIONS

More terms from Michel Marcus, Apr 10 2014

STATUS

approved