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 A067956 Number of nodes in virtual, "optimal", chordal graphs of diameter 4 and degree n+1. 1
 9, 16, 41, 66, 129, 192, 321, 450, 681, 912, 1289, 1666, 2241, 2816, 3649, 4482, 5641, 6800, 8361, 9922, 11969, 14016, 16641, 19266, 22569, 25872, 29961, 34050, 39041, 44032, 50049, 56066, 63241, 70416, 78889 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 REFERENCES Concrete Mathematics, R. L. Graham, D. E. Knuth, O. Patashnik, 1994, Addison-Wesley Company, Inc. LINKS Michael De Vlieger, Table of n, a(n) for n = 1..10000 FORMULA For n odd, t = (n+1)/2, a(n) = ((2*t*(t+1)*(t^2+t+4))/3)+1; for n even, t = n/2, a(n) = (((2*t*(t+1)*(t^2+t+4))/3)+1)+((2*t+1)*(2*t^2+2*t+3))/3. EXAMPLE For n=5, n is odd; t=3; a(5) = (2*3*(3+1)*(3^2+3+4)/3)+1 = ((6*4*16)/3)+1 = 129. For n=6, n is even; t=3; a(6) = a(5) + ((2*3+1)*(2*t^2+2*t+3))/3 = 129 + (7*27)/3 = 192. MAPLE for n from 1 to k do if ((n mod 2 ) = 1) then t := (n+1)/2; a[n] := ((2*(t*(t+1)*(t^2+t+4))/3)+1); else t := (n)/2; a[n] := ((2*(t*(t+1)*(t^2+t+4)/3)+1)+(2*t+1)*(2*t^2+2*t+3)/3); fi; print(a[n]); od; MATHEMATICA Array[((2 #2 (#2 + 1) (#2^2 + #2 + 4))/3) + 1 + (Boole[EvenQ[#1]]*((2 #2 + 1) (2 #2^2 + 2 #2 + 3))/3) & @@ {#, (# + Boole[OddQ[#]])/2} &, 35] (* Michael De Vlieger, Jul 29 2022 *) CROSSREFS Cf. A006007. Sequence in context: A309273 A358627 A282470 * A270757 A203595 A187087 Adjacent sequences: A067953 A067954 A067955 * A067957 A067958 A067959 KEYWORD nonn AUTHOR S. Bujnowski & B. Dubalski (slawb(AT)atr.bydgoszcz.pl), Mar 08 2002 STATUS approved

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Last modified May 30 06:53 EDT 2023. Contains 363045 sequences. (Running on oeis4.)