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 A119972 Flag n when the first difference of the decimal encoding of the Gray code is negative. 4
 1, 2, -3, 4, 5, -6, -7, 8, 9, 10, -11, -12, 13, -14, -15, 16, 17, 18, -19, 20, 21, -22, -23, -24, 25, 26, -27, -28, 29, -30, -31, 32, 33, 34, -35, 36, 37, -38, -39, 40, 41, 42, -43, -44, 45, -46, -47, -48, 49, 50, -51, 52, 53, -54, -55, -56, 57, 58, -59, -60, 61, -62, -63, 64, 65, 66, -67, 68, 69, -70, -71, 72, 73, 74, -75 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 LINKS FORMULA Merge A091072 with minus A091067 maintaining increasing absolute value. a(n) = n*Kronecker(-1, n) = A000027(n) * A034947(n). - Andrew Howroyd, Aug 06 2018 EXAMPLE A003188 begins 0 1 3 2 6 7 5 4 12 13 15 14 10 11 9 8 24 ...so A055975 begins 1 2-1 4 1-2-1 8 1 2 -1 -4 1 -2-1 16 ... so the negative positions are located at 3,6,7,11,12,14,..., = A091067 and the positive positions at the complement of A091067 which is A091072. MAPLE isA091067 := proc(n) option remember ; if n mod 4 = 3 then RETURN(true) ; else if n mod 2 = 0 then if isA091067(n/2) then RETURN(true) ; fi ; fi ; RETURN(false) ; fi ; end: A119972 := proc(n) if isA091067(n) then -n ; else n ; fi ; end: for n from 1 to 180 do printf("%d, ", A119972(n)) ; od ; # R. J. Mathar, May 14 2007 MATHEMATICA a[n_] := n KroneckerSymbol[-1, n]; Array[a, 75] (* Jean-François Alcover, Apr 09 2020 *) PROG (PARI) a(n) = n*kronecker(-1, n); \\ Andrew Howroyd, Aug 06 2018 CROSSREFS Cf. A000027, A034947, A003188, A055975, A091067, A091072. Sequence in context: A099570 A114142 A020725 * A131738 A199969 A303502 Adjacent sequences:  A119969 A119970 A119971 * A119973 A119974 A119975 KEYWORD easy,sign,mult AUTHOR Alford Arnold, Jun 01 2006 EXTENSIONS More terms from R. J. Mathar, May 14 2007 Keyword:mult added by Andrew Howroyd, Aug 06 2018 STATUS approved

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Last modified July 31 05:24 EDT 2021. Contains 346367 sequences. (Running on oeis4.)