login
Characteristic function of four.
16

%I #21 Jun 19 2024 09:15:23

%S 0,0,0,0,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,

%T 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,

%U 0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0

%N Characteristic function of four.

%C Number of connected 2-regular (simple) graphs with girth exactly 4.

%C The Euler transformation of this sequence is A121262.

%H <a href="/index/Rec#order_01">Index entries for linear recurrences with constant coefficients</a>, signature (1).

%F a(n) = A185114(n) - A185115(n).

%o (Python)

%o def A185014(n): return int(n==4) # _Chai Wah Wu_, Feb 04 2022

%Y Characteristic function of g: A000007 (g=0), A063524 (g=1), A185012 (g=2), A185013 (g=3), this sequence (g=4), A185015 (g=5), A185016 (g=6), A185017 (g=7).

%K nonn,easy

%O 0,1

%A _Jason Kimberley_, Sep 30 2011