1,1

This sequence and A000379 (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.

Contains (for example) 180, so is different from A123193. - Max Alekseyev, Sep 20 2007

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

From Vladimir Shevelev, Oct 28 2013: (Start)

Numbers n such that infinitary Moebius function of n (A064179) equals -1. This follows from the definition of A064179.

Number n is in the sequence if and only if the number k=k(n) of terms of A050376 which divide n with odd maximal exponent is odd (see example).

(End)

J. Lambek and L. Moser, On some two way classifications of integers, Canad. Math. Bull. 2 (1959), 85-89.

J. 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).

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.

If n=96, then the maximal exponent of 2 that divides 96 is 5, for 3 it is 1, for 4 it is 2, for 16 it is 1. Thus k(96)=3 and 96 is a term. - Vladimir Shevelev, Oct 28 2013

(Maple program from N. J. A. Sloane, Dec 20 2007) expts:=proc(n) local t1, t2, t3, t4, i; if n=1 then RETURN([0]); fi; if isprime(n) then RETURN([1]); fi; t1:=ifactor(n); if nops(factorset(n))=1 then RETURN([op(2, t1)]); fi; t2:=nops(t1); t3:=[]; for i from 1 to t2 do t4:=op(i, t1); if nops(t4) = 1 then t3:=[op(t3), 1]; else t3:=[op(t3), op(2, t4)]; fi; od; RETURN(t3); end; # returns a list of the exponents e_1, e_2, ...

A000120 := proc(n) local w, m, i; w := 0; m := n; while m > 0 do i := m mod 2; w := w+i; m := (m-i)/2; od; w; end: # returns weight of binary expansion

LamMos:= proc(n) local t1, t2, t3, i; t1:=expts(n); add( A000120(t1[i]), i=1..nops(t1)); end; # returns sum of weights of exponents

M:=400; t0:=[]; t1:=[]; for n from 1 to M do if LamMos(n) mod 2 = 0 then t0:=[op(t0), n] else t1:=[op(t1), n]; fi; od: t0; t1; # t0 is A000379, t1 is the present sequence

iMoebiusMu[ n_ ] := Switch[ MoebiusMu[ n ], 1, 1, -1, -1, 0, If[ OddQ[ Plus@@ (DigitCount[ Last[ Transpose[ FactorInteger[ n ] ] ], 2, 1 ]) ], -1, 1 ] ]; q=Select[ Range[ 20000 ], iMoebiusMu[ # ]===-1& ] (* Wouter Meeussen, Dec 21 2007 *)

Rest[Select[Range[150], OddQ[Count[Flatten[IntegerDigits[#, 2]&/@ Transpose[ FactorInteger[#]][[2]]], 1]]&]] (* Harvey P. Dale, Feb 25 2012 *)

(Haskell)

a000028 n = a000028_list !! (n-1)

a000028_list = filter (odd . sum . map a000120 . a124010_row) [1..]

-- Reinhard Zumkeller, Oct 05 2011

(PARI) is(n)=my(f=factor(n)[, 2]); sum(i=1, #f, hammingweight(f[i]))%2 \\ Charles R Greathouse IV, Aug 31 2013

Cf. A133008, A000379 (complement), A000120 (binary weight function), A064547; also A066724, A026477, A050376, A084400.

Note that A000069 and A001969, also A000201 and A001950 give other decompositions of the integers into two classes.

Cf. A124010 (prime exponents).

Sequence in context: A173345 A226091 A064175 * A026416 A123193 A066724

Adjacent sequences:  A000025 A000026 A000027 * A000029 A000030 A000031

nonn,nice,easy

N. J. A. Sloane, Simon Plouffe

Entry revised by N. J. A. Sloane, Dec 20 2007, restoring the original definition, correcting the entries and adding a new b-file.

approved