%I #18 Feb 29 2020 11:57:20

%S 1,3,8,24,86,348,1478,6324,26846,112668,467798,1925124,7867406,

%T 31980588,129475718,522603924,2104600766,8461122108,33972973238,

%U 136278002724,546271650926

%N Let P(A) be the power set of an n-element set A. Then a(n) = the number of pairs of elements {x,y} of P(A) for which either 0) x and y are disjoint and for which either x is a subset of y or y is a subset of x, or 1) x and y are disjoint and for which x is not a subset of y and y is not a subset of x, or 2) x and y are intersecting but for which x is not a subset of y and y is not a subset of x, or 3) x = y.

%H Ross La Haye, <a href="https://cs.uwaterloo.ca/journals/JIS/VOL12/LaHaye/lahaye5.html">Binary Relations on the Power Set of an n-Element Set</a>, Journal of Integer Sequences, Vol. 12 (2009), Article 09.2.6.

%H <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (10,-35,50,-24).

%F a(n) = (1/2)(4^n - 2*3^n + 5*2^n - 2) = 3*StirlingS2(n+1,4) + StirlingS2(n+1,3) + 2*StirlingS2(n+1,2) + 1.

%F G.f.: (1-7*x+13*x^2-x^3)/((1-x)*(1-2*x)*(1-3*x)*(1-4*x)). [_Colin Barker_, Jul 30 2012]

%e a(2) = 8 because for P(A) = {{},{1},{2},{1,2}} we have for case 0 {{},{1}}, {{},{2}}, {{},{1,2}} and we have for case 1 {{1},{2}} and we have for case 3 {{},{}}, {{1},{1}}, {{2},{2}}, {{1,2},{1,2}}. There are 0 {x,y} of P(A) in this example that fall under case 2.

%t LinearRecurrence[{10,-35,50,-24},{1,3,8,24},30] (* _Harvey P. Dale_, Feb 29 2020 *)

%Y Cf. A000225, A000392, A032263, A000079.

%K nonn,easy

%O 0,2

%A _Ross La Haye_, Jan 12 2008