A121758 - OEIS

%I #13 Jun 20 2017 12:24:38

%S -1,2,-3,4,-5,6,-7,8,-9,-10,-11,-8,-13,-6,-15,-4,-17,-2,-19,20,19,22,

%T 17,24,15,26,13,28,11,-30,-31,-28,-33,-26,-35,-24,-37,-22,-39,40,39,

%U 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

%N In decimal number system, take odd digits of n with negative sign.

%C See A121759. In decimal number system, take even digits of n with negative sign.

%C a(n) = - A121759(n). [_Reinhard Zumkeller_, Sep 16 2011]

%H Reinhard Zumkeller, <a href="/A121758/b121758.txt">Table of n, a(n) for n = 1..10000</a>

%F If n = d(i)*10^(i-1), then a(n)=(-1)^d(i)*d(i)*10^(i-1).

%e 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.

%t a[n_] := Total[MapIndexed[(-1)^#1*#1*10^(#2[[1]]-1)&, Reverse[ IntegerDigits[n] ] ] ]; Array[a, 76] (* _Jean-François Alcover_, Jun 20 2017 *)

%o (Haskell)

%o import Data.List (unfoldr)

%o a121758 = foldl (\v d -> 10 * v + d) 0 . reverse . unfoldr f where

%o    f 0 = Nothing

%o    f x | odd d     = Just (- d, x')

%o        | otherwise = Just (d, x') where (x', d) = divMod x 10

%o -- _Reinhard Zumkeller_, Sep 16 2011

%Y Cf. A121759.

%K base,sign,nice,look

%O 1,2

%A _Zak Seidov_, Aug 20 2006