This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A121759 #18 Jun 20 2017 12:24:30 %S A121759 1,-2,3,-4,5,-6,7,-8,9,10,11,8,13,6,15,4,17,2,19,-20,-19,-22,-17,-24, %T A121759 -15,-26,-13,-28,-11,30,31,28,33,26,35,24,37,22,39,-40,-39,-42,-37, %U A121759 -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,-51,70,71,68,73,66,75,64,77,62 %N A121759 In decimal number system, take even digits of n with negative sign. %C A121759 See A121758. In decimal number system, take odd digits of n with negative sign. %C A121759 a(A014261(n)) = A014261(n). - _Reinhard Zumkeller_, Nov 30 2007 %H A121759 Reinhard Zumkeller, <a href="/A121759/b121759.txt">Table of n, a(n) for n = 1..10000</a> %F A121759 If n = d(i)*10^(i-1), then a(n) = (-1)^(1+d(i))*d(i)*10^(i-1). %F A121759 a(n) = -A121758(n). %e A121759 a(12) = 8 because 12 = 1*10^1 + 2*10^0 and a(12) = (-1)^(1+1)*1*10^1 + (-1)^(1+2)*2*10^0 = 10-2 = 8. %t A121759 a[n_] := Total[MapIndexed[(-1)^(#1 + 1)*#1*10^(#2[[1]] - 1)&, Reverse[ IntegerDigits[n]]]]; Array[a, 78] (* _Jean-François Alcover_, Jun 20 2017 *) %o A121759 (Haskell) %o A121759 import Data.List (unfoldr) %o A121759 a121759 = foldl f 0 . reverse . unfoldr d where %o A121759 d 0 = Nothing %o A121759 d x = Just $ swap $ divMod x 10 %o A121759 f v d | even d = 10 * v - d %o A121759 | odd d = 10 * v + d %o A121759 -- _Reinhard Zumkeller_, Aug 01 2011 %Y A121759 Cf. A121758. %K A121759 base,sign,nice,look %O A121759 1,2 %A A121759 _Zak Seidov_, Aug 20 2006