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 A051952 Numbers that are not a sum of 3 positive squares nor are of the form 4^a*(8b+7) and which are not multiples of 4. 5
 1, 2, 5, 10, 13, 25, 37, 58, 85, 130 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS The asymptotic eigenvalue spectrum of the Schroedinger equation for a free particle in a box in three dimensions is known only (that is: average level density and average degeneracy) if the a(n) are finite series. It is not known whether 130 is the largest such number or if this is the start of an infinite series. See Theorem 6 of Grosswald's book, p. 79: every positive integer n not of the form 4^a*(8*m+7), with a and m nonnegative integers [i.e., n is a sum of three squares, proved by Legendre (1798)] and not of the form 4^b*a(n), with b a nonnegative integer, n=1,...,10, and possibly one a(11) > 5*10^10, is a sum of three non-vanishing squares. See A004214 and A000408. In the F. Halter-Koch reference, p. 13, one finds a Korollar 1 (b) stating for positive integers n, not 0, 4, 7 modulo 8: n is not the sum of three positive coprime squares if and only if n = a(n), n=1,...,10, and possibly one more number a(11) >= 5*10^10. See A223731. - Wolfdieter Lang, Apr 04 2013 REFERENCES E. Grosswald, Representations of Integers as Sums of Squares. Springer-Verlag, NY, 1985, p. 79 with p. 76. E. R. Hilf, Diploma-thesis, Univ.Frankfurt,Germany, 1963 [available from author] LINKS H. P. Baltes and E. R. Hilf, Spectra of finite systems; BI-Verlag. H.-P. Baltes, Peter K. J. Draxl and Eberhard R. Hilf, Quadratsummen und gewisse Randwertprobleme der Mathematischen Physik, Journ. Reine Angewandte Mathematik, Vol. 268/269, 1974, 410-417. P. K. J. Draxl, Sommes de deux carrés qui ne sont pas sommes de trois carrés strictement positifs, Mémoires de la Société Mathématique de France, 37 (1974), p. 53-53. E. Grosswald, A. Calloway and J. Calloway, The representations of integers by three positive squares, Proc. Amer. Math. Soc. 10 (1959), 451-455. [Math. Rev. 21 #3376; E24-73 in Leveque's Reviews in Number Theory, Vol. 2, p. 290] F. Halter-Koch, Darstellung natürlicher Zahlen als Summe von Quadraten, Acta Arith. 42 (1982) 11-20, p. 13. Eberhard R. Hilf, Publications E. R. Hilf and H. P. Baltes, 130 and the cube spectrum, unpublished E. R. Hilf, G. Suessmann, Surface Tension of nuclei according to the Fermi-gas-model, Physics Letters, Vol. 21, No. 6, p. 654-656, (1966) EXAMPLE Consider a(3)=5: 1^2 +1^2 +1^2=3, too low; 1^2+1^2+2^2=6, too high. 4^1=4 too low; 4^2=16 too high; (8*0+7)=7 too low, (8*1+7)= 15 too high; thus 5 is a member of this sequence. MATHEMATICA nmax = 1000; amax = Ceiling[Log[nmax/7]/Log[4]]; notThreeSquaresQ[n_] := Reduce[0 < a <= b <= c && n == a^2 + b^2 + c^2, {a, b, c}, Integers] === False; notOfTheFormQ[n_, a_] := Reduce[n == 4^a*(8*b+7), b, Integers] === False; notOfTheFormQ[n_] := And @@ (notOfTheFormQ[n, #] & ) /@ Range[0, amax]; Select[Range[nmax], !Mod[#, 4] == 0 && notThreeSquaresQ[#] && notOfTheFormQ[#] & ](* Jean-François Alcover, Jun 12 2012 *) CROSSREFS Sequence in context: A230550 A018571 A064233 * A103188 A281229 A185647 Adjacent sequences:  A051949 A051950 A051951 * A051953 A051954 A051955 KEYWORD nonn,hard,nice,more AUTHOR Eberhard R. Hilf (hilf(AT)merlin.physik.uni-oldenburg.de), Dec 21 1999 EXTENSIONS Grosswald et al. reference from N. J. A. Sloane, Jun 07 2000 STATUS approved

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Last modified July 22 20:51 EDT 2019. Contains 325226 sequences. (Running on oeis4.)