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A130517
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Triangle read by rows: row n counts down from n in steps of 2, then counts up the remaining elements in the set {1,2,...,n}, again in steps of 2.
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26
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1, 2, 1, 3, 1, 2, 4, 2, 1, 3, 5, 3, 1, 2, 4, 6, 4, 2, 1, 3, 5, 7, 5, 3, 1, 2, 4, 6, 8, 6, 4, 2, 1, 3, 5, 7, 9, 7, 5, 3, 1, 2, 4, 6, 8, 10, 8, 6, 4, 2, 1, 3, 5, 7, 9, 11, 9, 7, 5, 3, 1, 2, 4, 6, 8, 10, 12, 10, 8, 6, 4, 2, 1, 3, 5, 7, 9, 11, 13, 11, 9, 7, 5, 3, 1, 2, 4, 6, 8, 10, 12, 14, 12, 10
(list;
table;
graph;
refs;
listen;
history;
text;
internal format)
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OFFSET
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1,2
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COMMENTS
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Triangle read by rows in which row n lists the number of pairs of states of the subshells of the n-th shell of the nuclear shell model ordered by energy level in increasing order.
Row n lists a permutation of the first n positive integers.
If n is odd then row n lists the first (n+1)/2 odd numbers in decreasing order together with the first (n-1)/2 positive even numbers.
If n is even then row n lists the first n/2 even numbers in decreasing order together with the first n/2 odd numbers.
Row n >= 2, with its floor(n/2) last numbers taken as negative, lists the n different eigenvalues (in decreasing order) of the odd graph O(n). The odd graph O(n) has the (n-1)-subsets of a (2*n-1)-set as vertices, with two (n-1)-subsets adjacent if and only if they are disjoint. For example, O(3) is isomorphic to the Petersen graph. - Miquel A. Fiol, Apr 07 2024
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LINKS
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Eric Weisstein's World of Mathematics, Odd graph
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FORMULA
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T(1,1) = 1; for n > 1: T(n,1) = T(n-1,1)+1 and T(n,k) = T(n-1,n-k+1), 1 < k <= n. - Reinhard Zumkeller, Dec 03 2012
a(n) = |2*n - t^2 - 2*t - 3| + floor((2*n - t^2 - t)/(t+3)) where t = floor((-1+sqrt(8*n-7))/2). (End)
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EXAMPLE
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A geometric model of the atomic nucleus:
......-------------------------------------------------
......|...-----------------------------------------...|
......|...|...---------------------------------...|...|
......|...|...|...-------------------------...|...|...|
......|...|...|...|...-----------------...|...|...|...|
......|...|...|...|...|...---------...|...|...|...|...|
......|...|...|...|...|...|...-...|...|...|...|...|...|
......i...h...g...f...d...p...s...p...d...f...g...h...i
......|...|...|...|...|...|.......|...|...|...|...|...|
......|...|...|...|...|.......1.......|...|...|...|...|
......|...|...|...|.......2.......1.......|...|...|...|
......|...|...|.......3.......1.......2.......|...|...|
......|...|.......4.......2.......1.......3.......|...|
......|.......5.......3.......1.......2.......4.......|
..........6.......4.......2.......1.......3.......5....
......7.......5.......3.......1.......2.......4.......6
.......................................................
...13/2.11/2.9/2.7/2.5/2.3/2.1/2.1/2.3/2.5/2.7/2.9/2.11/2
......|...|...|...|...|...|...|...|...|...|...|...|...|
......|...|...|...|...|...|...-----...|...|...|...|...|
......|...|...|...|...|...-------------...|...|...|...|
......|...|...|...|...---------------------...|...|...|
......|...|...|...-----------------------------...|...|
......|...|...-------------------------------------...|
......|...---------------------------------------------
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Triangle begins:
1;
2, 1;
3, 1, 2;
4, 2, 1, 3;
5, 3, 1, 2, 4;
6, 4, 2, 1, 3, 5;
7, 5, 3, 1, 2, 4, 6;
8, 6, 4, 2, 1, 3, 5, 7;
9, 7, 5, 3, 1, 2, 4, 6, 8;
10, 8, 6, 4, 2, 1, 3, 5, 7, 9;
...
Also:
1;
2, 1;
3, 1, 2;
4, 2, 1, 3;
5, 3, 1, 2, 4;
6, 4, 2, 1, 3, 5;
7, 5, 3, 1, 2, 4, 6;
8, 6, 4, 2, 1, 3, 5, 7;
9, 7, 5, 3, 1, 2, 4, 6, 8;
10, 8, 6, 4, 2, 1, 3, 5, 7, 9;
...
In this view each column contains the same numbers.
Eigenvalues of the odd graphs O(n) for n=2..10:
2, -1;
3, 1, -2;
4, 2, -1, -3;
5, 3, 1, -2, -4;
6, 4, 2, -1, -3, -5;
7, 5, 3, 1, -2, -4, -6;
8, 6, 4, 2, -1, -3, -5, -7;
9, 7, 5, 3, 1, -2, -4, -6, -8;
10, 8, 6, 4, 2, -1, -3, -5, -7, -9;
... (End)
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MAPLE
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if k <= (n+1)/2 then
n-2*(k-1) ;
else
1-n+2*(k-1) ;
end if;
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MATHEMATICA
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t[n_, 1] := n; t[n_, n_] := n-1; t[n_, k_] := Abs[2*k-n - If[2*k <= n+1, 2, 1]]; Table[t[n, k], {n, 1, 14}, {k, 1, n}] // Flatten (* Jean-François Alcover, Oct 03 2013, from abs(A056951) *)
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PROG
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(Haskell)
a130517 n k = a130517_tabl !! (n-1) !! (k-1)
a130517_row n = a130517_tabl !! (n-1)
a130517_tabl = iterate (\row -> (head row + 1) : reverse row) [1]
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CROSSREFS
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KEYWORD
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AUTHOR
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STATUS
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approved
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