Dave R.M. Langers has authored 6 sequences.
A357811
Number of n-step closed paths on the Cairo pentagonal lattice graph starting from a degree-3 node.
Original entry on oeis.org
1, 0, 3, 0, 17, 6, 115, 100, 867, 1236, 7117, 13770, 62545, 146866, 579387, 1537920, 5581725, 16002810, 55329435, 166465820, 559913787, 1736268432, 5752600961, 18182999274, 59777071435, 191287075320
Offset: 0
For n=2, the a(2) = 3 solutions visit one of the three vertices adjacent to the initial vertex, and then return.
For n=5, the a(5) = 6 solutions consist of counterclockwise or clockwise traversals of the circumference of any of the three pentagons that surround the initial vertex.
The accompanying sequences for the number of paths that return to a degree-4 node is
A357810.
A357810
Number of n-step closed paths on the Cairo pentagonal lattice graph starting from a degree-4 node.
Original entry on oeis.org
1, 0, 4, 0, 24, 8, 164, 136, 1236, 1704, 10116, 19144, 88616, 205208, 818764, 2155160, 7873440, 22463400, 77954740, 233894600, 788314984, 2440865400, 8095906076, 25569342520, 84107990356, 269034666280
Offset: 0
For n=2, the a(2) = 4 solutions visit one of the four vertices adjacent to the initial vertex, and then return.
For n=5, the a(5) = 8 solutions consist of counterclockwise or clockwise traversals of the circumference of any of the four pentagons that surround the initial vertex.
The accompanying sequences for the number of paths that return to a degree-3 node is
A357811.
A357771
Number of 2n-step closed paths on quasi-regular rhombic (rhombille) lattice starting from a degree-6 node.
Original entry on oeis.org
1, 6, 60, 744, 10224, 148896, 2250816, 34922880, 552386304, 8867756544, 144044098560, 2362292213760, 39049785446400, 649843233546240, 10876273137008640, 182934715370471424, 3090181365862170624, 52398620697685524480, 891492911924665122816, 15213249205591283859456, 260315328935885892747264
Offset: 0
a(2)=60, because there are 6*6=36 paths that visit one of six adjacent vertices, return to the origin, and again visit an adjacent vertex and return to the origin; plus 6*4=24 paths that pass through one of the six vertices at distance 2, leaving and returning via any of two available paths to that vertex; all resulting in a closed path of length 2n=2*2=4.
The accompanying sequences for the number of paths that return to a degree-3 node is
A357770.
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a[n_] := Sum[Binomial[n, k]*Sum[Binomial[n, j]*Sum[(1/(2^j))*Binomial[2*i, j]*Binomial[2*i, i]*Binomial[2*(j-i), j-i],{i,0,j}],{j,0,n}],{k,0,n}]; Flatten[Table[a[n],{n,0,17}]] (* Detlef Meya, May 15 2024 *)
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a(n) = sum(k=0, n, binomial(n, k) * sum(j=0, n, binomial(n, j) * sum(i=0, j, (1/(2^j)*binomial(2*i, j)*binomial(2*i, i)*binomial(2*(j-i), j-i))))); \\ Michel Marcus, May 20 2024
A357770
Number of 2n-step closed paths on quasi-regular rhombic (rhombille) lattice starting from a degree-3 node.
Original entry on oeis.org
1, 3, 30, 372, 5112, 74448, 1125408, 17461440, 276193152, 4433878272, 72022049280, 1181146106880, 19524892723200, 324921616773120, 5438136568504320, 91467357685235712, 1545090682931085312, 26199310348842762240, 445746455962332561408, 7606624602795641929728
Offset: 0
a(2)=30, because there are 3*3=9 paths that visit one of three adjacent vertices, return to the origin, and again visit an adjacent vertex and return to the origin; 3*5=15 paths visiting one of five distance-2 vertices that are adjacent to the three adjacent vertices; plus 3*2=6 paths traversing the perimeter of three adjacent rhombi in counterclockwise or clockwise direction; all resulting in a closed path of length 2n=2*2=4.
The accompanying sequences for the number of paths that return to a degree-6 node is
A357771.
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a[0] := 1; a[n_] := Sum[Binomial[n, k]*Sum[Binomial[n, j]*Sum[(1/(2^(j+1)))*Binomial[2*i, j]*Binomial[2*i, i]*Binomial[2*(j-i), j-i], {i, 0, j}], {j, 0, n}], {k, 0, n}]; Flatten[Table[a[n], {n, 0, 19}]] (* Detlef Meya, May 20 2024 *)
A356258
Number of 6-dimensional cubic lattice walks that start and end at origin after 2n steps, free to pass through origin at intermediate stages.
Original entry on oeis.org
1, 12, 396, 19920, 1281420, 96807312, 8175770064, 748315668672, 72729762868620, 7402621930738320, 781429888276676496, 84955810313787521472, 9463540456205136873936, 1075903653146632508721600, 124461755084172965028753600, 14615050011682746903615601920
Offset: 0
a(1)=12, because twelve paths start at the origin, visit one of the adjacent vertices, and immediately return to the origin, resulting in 12 different paths of length 2n=2*1=2.
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b:= proc(n, i) option remember; `if`(n=0 or i=1, 1,
add(b(n-j, i-1)*binomial(n, j)^2, j=0..n))
end:
a:= n-> (2*n)!*b(n, 6)/n!^2:
seq(a(n), n=0..15); # Alois P. Heinz, Jan 30 2023
A232005
Number of distinct resistances that can be produced from a circuit of resistors with resistances 1, 2, ..., n using only series and parallel combinations.
Original entry on oeis.org
1, 2, 8, 48, 386, 3781, 49475, 762869, 13554897, 266817541
Offset: 1
a(2) = 2 since given a 1-ohm and a 2-ohm resistor, a series circuit yields 3 ohms, while a parallel circuit yields 2/3 ohms, which thus yields two distinct resistances.
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