A000379 Numbers where total number of 1-bits in the exponents of their prime factorization is even; a 2-way classification of integers: complement of A000028.

1, 6, 8, 10, 12, 14, 15, 18, 20, 21, 22, 26, 27, 28, 32, 33, 34, 35, 36, 38, 39, 44, 45, 46, 48, 50, 51, 52, 55, 57, 58, 62, 63, 64, 65, 68, 69, 74, 75, 76, 77, 80, 82, 85, 86, 87, 91, 92, 93, 94, 95, 98, 99, 100, 106, 111, 112, 115, 116, 117, 118, 119, 120, 122, 123, 124, 125, 129

Offset: 1

Author

N. J. A. Sloane

Keywords

• nonn
• easy
• nice

Comments

This sequence and A000028 (its complement) give the unique solution to the problem of splitting the positive integers into two classes in such a way that products of pairs of distinct elements from either class occur with the same multiplicities [Lambek and Moser]. Cf. A000069, A001969.

See A000028 for precise definition, Maple program, etc.

The sequence contains products of even number of distinct terms of A050376. - Vladimir Shevelev, May 04 2010

From Vladimir Shevelev, Oct 28 2013: (Start)

Numbers m such that the infinitary Möbius function (A064179) of m equals 1. (This follows from the definition of A064179.)

A number m is in the sequence iff the number k = k(m) of terms of A050376 that divide m with odd maximal exponent is even (see example).

(End)

Numbers k for which A064547(k) [or equally, A268386(k)] is even. Numbers k for which A010060(A268387(k)) = 0. - Antti Karttunen, Feb 09 2016

The sequence is closed under the commutative binary operation A059897(.,.). As integers are self-inverse under A059897(.,.), it therefore forms a subgroup of the positive integers considered as a group under A059897(.,.). Specifically (expanding on the comment above dated May 04 2010) it is the subgroup of even length words in A050376, which is the group's lexicographically earliest ordered minimal set of generators. A000028, the set of odd length words in A050376, is its complementary coset. - Peter Munn, Nov 01 2019

From Amiram Eldar, Oct 02 2024: (Start)

Numbers whose number of infinitary divisors (A037445) is a square.

Numbers whose exponentially odious part (A367514) has an even number of distinct prime factors, i.e., numbers k such that A092248(A367514(k)) = 0. (End)

Examples

			If m = 120, then the maximal exponent of 2 that divides 120 is 3, for 3 it is 1, for 4 it is 1, for 5 it is 1. Thus k(120) = 4 and 120 is a term. - _Vladimir Shevelev_, Oct 28 2013
		

References

• Joe Roberts, Lure of the Integers, Math. Assoc. America, 1992, p. 22.
• N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
• N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

Links

• N. J. A. Sloane, Table of n, a(n) for n = 1..10000
• J. Lambek and L. Moser, On some two way classifications of integers, Canad. Math. Bull. 2 (1959), 85-89.
• Eric Weisstein's World of Mathematics, Group.
• Wikipedia, Generating set of a group.

Crossrefs

Subsequences: A030229, A238748, A262675, A268390.

Subsequence of A268388 (apart from the initial 1).

Complement: A000028.

Sequences used in definitions of this sequence: A133008, A050376, A059897, A064179, A064547, A124010 (prime exponents), A268386, A268387, A010060.

Other 2-way classifications: A000069/A001969 (to which A000120 and A010060 are relevant), A000201/A001950.

This is different from A123240 (e.g., does not contain 180). The first difference occurs already at n=31, where A123240(31) = 60, a value which does not occur here, as a(31+1) = 62. The same is true with respect to A131181, as A131181(31) = 60.

Cf. A030231, A037445, A092248, A367514.

Programs

Haskell

a000379 n = a000379_list !! (n-1)
a000379_list = filter (even . sum . map a000120 . a124010_row) [1..]
-- Reinhard Zumkeller, Oct 05 2011

Mathematica

Select[ Range[130], EvenQ[ Count[ Flatten[ IntegerDigits[#, 2]& /@ Transpose[ FactorInteger[#]][[2]]], 1]]&] // Prepend[#, 1]& (* Jean-François Alcover, Apr 11 2013, after Harvey P. Dale *)

PARI

is(n)=my(f=factor(n)[,2]); sum(i=1,#f,hammingweight(f[i]))%2==0 \\ Charles R Greathouse IV, Aug 31 2013
(Scheme, two variants)
(define A000379 (MATCHING-POS 1 1 (COMPOSE even? A064547)))
(define A000379 (MATCHING-POS 1 1 (lambda (n) (even? (A000120 (A268387 n))))))
;; Both require also my IntSeq-library. - Antti Karttunen, Feb 09 2016

Extensions

Edited by N. J. A. Sloane, Dec 20 2007, to restore the original definition.