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A337566
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a(n) is the number of possible decompositions of the polynomial n * (x + x^2 + ... + x^q), where q > 1, into a sum of k polynomials, not necessarily all different; each of these polynomials is to be of the form b_1 * x + b_2 * x^2 + ... + b_q * x^q where each b_i is one of the numbers 1, 2, 3, ..., q and no two b_i are equal.
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1
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0, 0, 1, 1, 1, 3, 1, 2, 3, 3, 1, 5, 1, 3, 5, 3, 1, 6, 1, 5, 5, 3, 1, 7, 3, 3, 5, 5, 1, 9, 1, 4, 5, 3, 5, 9, 1, 3, 5, 7, 1, 9, 1, 5, 9, 3, 1, 9, 3, 6, 5, 5, 1, 9, 5, 7, 5, 3, 1, 13, 1, 3, 9, 5, 5, 9, 1, 5, 5, 9, 1, 12, 1, 3, 9, 5, 5, 9, 1, 9, 7, 3, 1, 13, 5, 3, 5, 7, 1, 15, 5, 5
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OFFSET
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1,6
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COMMENTS
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Inspired by the 6th problem of the 13th British Mathematical Olympiad in 1977 (see the link BMO) where the problem asked to find for n = 26 all the values of q for which this decomposition is possible (see 2nd example).
As mentioned by Tony Gardiner in his book (see reference), "the wording" of this problem "is very strange". Letter n in Olympiad exercise becomes q in the Name.
If a solution is the sum of k polynomials of degree q, then, the relation between (n,k,q) is: k*(q+1) = 2*n with q > 1 (as in the problem) and q < n (because one proves there is no solution when q >= n); then, a(n) is the number of pairs (k,q) that are solutions of this last relation.
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REFERENCES
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A. Gardiner, The Mathematical Olympiad Handbook: An Introduction to Problem Solving, Oxford University Press, 1997, reprinted 2011, Problem 6 pp. 212-213 (1977).
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LINKS
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British Mathematical Olympiad 1977, Problem 6.
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FORMULA
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a(1) = 0 then, for n >= 2, a(n) = tau(2n) - 3 = A099777(n) - 3.
a(n) = 1 iff n = 4 or n = p odd prime (A065091).
a(n) = p-3, p odd prime > 3 iff n = 2^(p-2).
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EXAMPLE
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For n = 3, the only solution, that corresponds to q = 2 and k = 2, is:
3 * (x + x^2) = (x + 2x^2) + (2x + x^2).
For n = 26 as in the British Olympiad problem, a(26) = 3, and these three possible decompositions are:
for k = 2, q = 25:
26 * (x + x^2 + x^3 + ... + x^24 + x^25) =
(x + 2x^2 + 3x^3 + ... + 24x^24 + 25x^25) +
(25x + 24x^2 + 23x^3 + ... + 2x^24 + x^25);
for k = 4, q = 12:
26 * (x + x^2 + x^3 + ... + x^11 + x^12) =
(x + 2x^2 + 3x^3 + ... + 11x^11 + 12x^12) +
(12x + 11x^2 + 10x^3 + ... + 2x^11 + x^12) +
(x + 2x^2 + 3x^3 + ... + 11x^11 + 12x^12) +
(12x + 11x^2 + 10x^3 + ... + 2x^11 + x^12);
for k = 13, q = 3:
26 * (x + x^2 + x^3) =
4 * (x + 2x^2 + 3x^3) +
4 * (2x + 3x^2 + x^3) +
3 * (3x + x^2 + 2x^3) +
(2x + x^2 + 3x^3) +
(3x + 2x^2 + x^3).
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MAPLE
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with(numtheory):
Data:= 0, seq(tau(2*n)-3, n=2..150);
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MATHEMATICA
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MapAt[# + 1 &, Array[DivisorSigma[0, 2 #] - 3 &, 92], 1] (* Michael De Vlieger, Dec 12 2021 *)
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PROG
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(PARI) a(n) = if (n==1, 0, numdiv(2*n)-3); \\ Michel Marcus, Sep 06 2020
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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