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 A121758 In decimal number system, take odd digits of n with negative sign. 2
 -1, 2, -3, 4, -5, 6, -7, 8, -9, -10, -11, -8, -13, -6, -15, -4, -17, -2, -19, 20, 19, 22, 17, 24, 15, 26, 13, 28, 11, -30, -31, -28, -33, -26, -35, -24, -37, -22, -39, 40, 39, 42, 37, 44, 35, 46, 33, 48, 31, -50, -51, -48, -53, -46, -55, -44, -57, -42, -59, 60, 59, 62, 57, 64, 55, 66, 53, 68, 51, -70, -71, -68, -73, -66, -75, -64 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS See A121759. In decimal number system, take even digits of n with negative sign. a(n) = - A121759(n). [Reinhard Zumkeller, Sep 16 2011] LINKS Reinhard Zumkeller, Table of n, a(n) for n = 1..10000 FORMULA If n = d(i)*10^(i-1), then a(n)=(-1)^d(i)*d(i)*10^(i-1). EXAMPLE a(12)=-8 because 12=1*10^1+2*10^0 and a(12)=(-1)^1*1*10^1+(-1)^2*2*10^0=-10+2=-8. MATHEMATICA a[n_] := Total[MapIndexed[(-1)^#1*#1*10^(#2[[1]]-1)&, Reverse[ IntegerDigits[n] ] ] ]; Array[a, 76] (* Jean-François Alcover, Jun 20 2017 *) PROG (Haskell) import Data.List (unfoldr) a121758 = foldl (\v d -> 10 * v + d) 0 . reverse . unfoldr f where    f 0 = Nothing    f x | odd d     = Just (- d, x')        | otherwise = Just (d, x') where (x', d) = divMod x 10 -- Reinhard Zumkeller, Sep 16 2011 CROSSREFS Cf. A121759. Sequence in context: A043271 A278063 A062759 * A121759 A265310 A263283 Adjacent sequences:  A121755 A121756 A121757 * A121759 A121760 A121761 KEYWORD base,sign,nice,look AUTHOR Zak Seidov, Aug 20 2006 STATUS approved

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