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A069999
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Number of possible dimensions for commutators of n X n matrices; it is independent of the field. Or, given a partition P = (p_1, p_2, ..., p_m) of n with p_1 >= p_2 >= ... >= p_m, let S(P) = sum_j (2j-1)p_j; then a(n) = number of integers that are an S(P) for some partition.
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8
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1, 1, 2, 3, 5, 7, 9, 13, 18, 21, 27, 34, 39, 46, 54, 61, 72, 83, 92, 106, 118, 130, 145, 162, 176, 193, 209, 226, 246, 265, 284, 308, 330, 352, 375, 402, 426, 453, 480, 508, 538, 570, 598, 631, 661, 694, 730, 765, 800, 835, 872, 911, 951, 992, 1030, 1071, 1115
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OFFSET
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0,3
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COMMENTS
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Or, given such a partition P of n, let T(P) = sum_i p_i^2; then a(n) = number of integers that are a T(P) for some P. While T(P) need not equal S(P) for a given partition, the two sets of integers are equal. Or, expand the infinite product prod_k 1/(1-x^{k^2}y^k) as a power series; then a(n) = number of terms of the form x^my^n having a nonzero coefficient.
The least m for which there are distinct partitions x(1)+...+x(k) of n for which the sums of squares {x(i)^2} are not distinct is 6. [Clark Kimberling, Mar 06 2012]
a(n) is also the number of possible counts of intersection points of n lines in the plane, no three concurrent. This is because n lines, grouped into pencils of size a_1,...,a_k, meet in P=Sum_{i<j} a_i a_j points, and such sums P are bijective with sums of squares S=a_1^2+...+a_k^2, thanks to n^2=S+2P. For example, a(10)=27 since 10 lines can meet in 0, 9, 16, 17, 21, 23, 24, 25 or 27..45 points. [Alon Amit, May 20 2019]
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REFERENCES
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Zachary Albertson and Evan Willett, "Possible Dimensions of Commutators of Matrices", Senior Thesis, Wake Forest University, May 09, 2002.
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LINKS
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FORMULA
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No generating function is known.
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MAPLE
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b:= proc(n, i) option remember; `if`(n=0 or i=1, {n},
{b(n, i-1)[], map(x-> x+i^2, b(n-i, min(n-i, i)))[]})
end:
a:= n-> nops(b(n$2)):
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MATHEMATICA
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p[n_, k_] := (IntegerPartitions[n]^2)[[k]]; s[n_, k_] := Sum[p[n, k][[i]], {i, 1, Length[p[n, k]]}]; t = Table[s[n, k], {n, 1, 20}, {k, 1, Length[IntegerPartitions[n]]}]; Table[Length[Union[t[[n]]]], {n, 1, 20}] (* Clark Kimberling, Mar 06 2012 *)
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PROG
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(PARI)
a069999(N)= \\ terms up to a(N), b-file format
{
my( V = vector(N) );
V[1] = 'x;
print(1, " ", 1 );
for (j=2, N,
my( t = x^(j*j) );
for (a=1, j-1,
my( b = j - a );
if ( a > b, break() );
t += V[a] * V[b];
);
t = Pol( apply( x->x!=0, Vec(t) ) );
print(j, " ", vecsum( Vec(t) ) );
V[j] = t;
);
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CROSSREFS
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KEYWORD
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easy,nonn,nice
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AUTHOR
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Jim Kuzmanovich (kuz(AT)wfu.edu), Apr 26 2002
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EXTENSIONS
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STATUS
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approved
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