%I #27 Feb 23 2023 09:31:56

%S 2,1,0,-1,-2,-3,-4,-5,-6,-7,-8,-9,-10,-11,-12,-13,-14,-15,-16,-17,-18,

%T -19,-20,-21,-22,-23,-24,-25,-26,-27,-28,-29,-30,-31,-32,-33,-34,-35,

%U -36,-37,-38,-39,-40,-41,-42,-43,-44,-45,-46,-47,-48,-49,-50,-51,-52,-53,-54,-55

%N a(n) = 2 - n.

%C a(n) is the Euler characteristic of one-sided surface of genus n (see Courant and Herbert). - _Stefano Spezia_, Sep 10 2022

%H Richard Courant and Herbert Robbins, <a href="https://archive.org/details/in.ernet.dli.2015.55043/page/n283/mode/2up">What Is Mathematics?</a>, Oxford, 1941, p. 262.

%H Tanya Khovanova, <a href="http://www.tanyakhovanova.com/RecursiveSequences/RecursiveSequences.html">Recursive Sequences</a>

%H <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (2,-1).

%F From _Paul Barry_, Mar 31 2007: (Start)

%F G.f.: (2-3x)/(1-x)^2.

%F E.g.f.: exp(x)*(2-x). (End)

%F a(n) = 2*a(n-1) - a(n-2); a(0)=2, a(1)=1. - _Philippe Deléham_, Nov 03 2008

%t 2-Range[0,60] (* or *) LinearRecurrence[{2,-1},{2,1},60] (* _Harvey P. Dale_, Feb 23 2023 *)

%o (PARI) a(n)=n-2 \\ _Charles R Greathouse IV_, Jun 11 2015

%Y Cf. A239229.

%K sign,easy

%O 0,1

%A _N. J. A. Sloane_