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 A340129 a(n) is the number of solutions of the Diophantine equation x^2 + y^2 = z^5 + z, gcd(x, y, z) = 1, x <= y, where z = A008784(n). 2
 1, 1, 2, 4, 2, 2, 4, 4, 2, 2, 4, 4, 4, 4, 4, 2, 4, 2, 2, 2, 16, 4, 4, 4, 2, 4, 2, 4, 4, 8, 8, 8, 4, 4, 4, 2, 8, 4, 16, 4, 16, 4, 2, 4, 16, 4, 4, 16, 4, 8, 8, 8, 4, 4, 8, 4, 8, 4, 4, 4, 16, 4, 4, 8, 2, 16, 2, 32, 2, 16, 4, 4, 2, 4, 8, 16, 4, 8, 4, 8, 4, 4, 8, 4, 16 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS The idea of this sequence comes from the 6th problem of the 21st British Mathematical Olympiad in 1985 where it is asked to show that this equation has infinitely many solutions (see link Olympiads and reference Gardiner). Indeed, this Diophantine equation x^2 + y^2 = z^5 + z with gcd(x, y, z) = 1 has solutions iff z is in A008784. When z is in A008784, there exist (u, v), gcd(u, v) = 1 such that z = u^2 + v^2; then, (u*z^2-v)^2 + (u+v*z^2)^2 = z^5 + z. Hence, with x = min(u*z^2-v, u+v*z^2) and y = max(u*z^2-v, u+v*z^2), the equation x^2 + y^2 = z^5 + z is satisfied. So this equation has infinitely many solutions since it has at least one solution for each term of A008784. For instance, for z = 10 we have:   with (u,v) = (1,3), then x = 1*10^2-3 = 97 and y = 1+3*10^2 = 301,   with (u,v) = (3,1), then x = 3+1*10^2 = 103 and y = 3*10^2-1 = 299,   so finally 97^2 + 301^2 = 103^2 + 299^2 = 10^5 + 10. Note that some z, among them 10, have other solutions not of this form. REFERENCES A. Gardiner, The Mathematical Olympiad Handbook: An Introduction to Problem Solving, Oxford University Press, 1997, reprinted 2011, Problem 6, pp. 63 and 167-168 (1985). LINKS Amiram Eldar, Table of n, a(n) for n = 1..10000 British Mathematical Olympiad, 1985 - Problem 6. EXAMPLE For z = A008784(1) = 1, 1^2 + 1^2 = 1^5 + 1 is the only solution, so a(1) = 1. For z = A008784(3) = 5, 23^2 + 51^2 = 27^2 + 49^2 = 5^5 + 5 so a(3) = 2. For z = A008784(4) = 10, (97, 301, 10), (103, 299, 10), (119, 293, 10) and (163, 271, 10) are solutions, so a(4) = 4. MATHEMATICA f[n_] := Length @ Solve[x^2 + y^2 == n^5 + n && GCD @@ {x, y, n} == 1 && 0 <= x <= y, {x, y}, Integers]; f /@ Select[Range, IntegerExponent[#, 2] < 2 && AllTrue[FactorInteger[#][[;; , 1]], Mod[#1, 4] < 3 &] &] (* Amiram Eldar, Jan 22 2021 *) PROG (PARI) f(z) = {if (issquare(Mod(-1, z)), my(nb = 0, s = z^5+z, d, j); for (i=1, sqrtint(s), if (issquare(d = s - i^2), j = sqrtint(d); if ((j<=i) && gcd([i, j, z]) == 1, nb++); ); ); nb; ); } lista(nn) = {for (n=1, nn, if (issquare(Mod(-1, n)), print1(f(n), ", ")); ); } \\ Michel Marcus, Jan 20 2021 CROSSREFS Cf. A008784, A048147, A131471. Sequence in context: A021809 A210210 A117007 * A213433 A294354 A144049 Adjacent sequences:  A340126 A340127 A340128 * A340130 A340131 A340132 KEYWORD nonn AUTHOR Bernard Schott, Jan 17 2021 EXTENSIONS More terms from Michel Marcus, Jan 20 2021 STATUS approved

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Last modified October 23 00:39 EDT 2021. Contains 348211 sequences. (Running on oeis4.)