OFFSET
1,2
COMMENTS
Each vertex can dominate up to three others. A ladder with a length that is an exact multiple of three can be dominated in only one way with 2n/3 vertices. - Andrew Howroyd, Apr 21 2018
LINKS
Andrew Howroyd, Table of n, a(n) for n = 1..200
Eric Weisstein's World of Mathematics, Ladder Graph
Eric Weisstein's World of Mathematics, Total Dominating Set
Index entries for linear recurrences with constant coefficients, signature (0,0,5,0,0,-10,0,0,10,0,0,-5,0,0,1).
FORMULA
a(n) = 1 for n mod 3 = 0
= ((n^2 + 13*n + 4)/18)^2 for n mod 3 = 1
= ((n + 4)/3)^2 for n mod 3 = 2.
G.f.: x*(-1 - 4*x - x^2 - 11*x^3 + 11*x^4 + 4*x^5 + 6*x^6 - 11*x^7 - 6*x^8 + x^9 + 5*x^10 + 4*x^11 - x^12 - x^13 - x^14)/(-1 + x^3)^5.
a(n) = 5*a(n-3) - 10*a(n-6) + 10*a(n-9) - 5*a(n-12) + a(n-15) for n>15. - Colin Barker, Apr 23 2018
EXAMPLE
From Andrew Howroyd, Apr 21 2018: (Start)
a(9) = 1 because there is only one arrangement of 6 vertices that is totally dominating and no set with fewer vertices can be totally dominating:
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(End)
MATHEMATICA
Table[Piecewise[{{1, Mod[n, 3] == 0}, {((n^2 + 13 n + 4)/18)^2, Mod[n, 3] == 1}, {((n + 4)/3)^2, Mod[n, 3] == 2}}], {n, 58}] (* Eric W. Weisstein, Apr 23 2018 and Michael De Vlieger, Apr 21 2018 *)
Table[(916 + 392 n + 213 n^2 + 26 n^3 + n^4 - (-56 + 392 n + 213 n^2 + 26 n^3 + n^4) Cos[2 n Pi/3] + Sqrt[3] (-20 + 7 n + n^2) (28 + 19 n + n^2) Sin[2 n Pi/3])/972, {n, 20}] (* Eric W. Weisstein, Apr 23 2018 *)
LinearRecurrence[{0, 0, 5, 0, 0, -10, 0, 0, 10, 0, 0, -5, 0, 0, 1}, {1, 4, 1, 16, 9, 1, 64, 16, 1, 169, 25, 1, 361, 36, 1}, 20] (* Eric W. Weisstein, Apr 23 2018 *)
CoefficientList[Series[(-1 - 4 x - x^2 - 11 x^3 + 11 x^4 + 4 x^5 + 6 x^6 - 11 x^7 - 6 x^8 + x^9 + 5 x^10 + 4 x^11 - x^12 - x^13 - x^14)/(-1 + x^3)^5, {x, 0, 20}], x] (* Eric W. Weisstein, Apr 23 2018 *)
PROG
(PARI) a(n)={if(n%3==0, 1, if(n%3==1, (n^2 + 13*n + 4)/18, (n + 4)/3))^2} \\ Andrew Howroyd, Apr 21 2018
(PARI) Vec(x*(1 + 4*x + x^2 + 11*x^3 - 11*x^4 - 4*x^5 - 6*x^6 + 11*x^7 + 6*x^8 - x^9 - 5*x^10 - 4*x^11 + x^12 + x^13 + x^14) / ((1 - x)^5*(1 + x + x^2)^5) + O(x^60)) \\ Colin Barker, Apr 23 2018
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Eric W. Weisstein, Apr 17 2018
EXTENSIONS
Terms a(14) and beyond from Andrew Howroyd, Apr 21 2018
STATUS
approved