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A050801 Numbers n such that n^2 is expressible as the sum of two positive cubes in at least one way. 10
3, 4, 24, 32, 81, 98, 108, 168, 192, 228, 256, 312, 375, 500, 525, 588, 648, 671, 784, 847, 864, 1014, 1029, 1183, 1225, 1261, 1323, 1344, 1372, 1536, 1824, 2048, 2187, 2496, 2646, 2888, 2916, 3000, 3993, 4000, 4200, 4225, 4536, 4563, 4644, 4704, 5184, 5324 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Analogous solutions exist for the sum of two identical cubes z^2 = 2*r^3 (e.g. 864^2 = 2*72^3). Values of 'z' are the terms in A033430, values of 'r' are the terms in A001105.

First term that can be expressed in two ways: 77976^2 = 228^3+1824^3 = 1026^3+1710^3. - Jud McCranie.

First term that can be expressed in three ways: 3343221000^2 = 279300^3 + 2234400^3 = 790020^3 + 2202480^3 = 1256850^3 + 2094750^3.

First term that can be expressed in four ways <= 42794271007595289 where 42794271007595289^2 = 14385864402^3 + 122279847417^3 = 55172161278^3 + 118485773289^3 = 64117642953^3 + 116169722214^3 = 96704977369^3 + 97504192058^3.

First term that can be expressed in five ways <= 47155572445935012696000 where 47155572445935012696000^2 = 94405759361550^3 + 1305070263601650^3 = 374224408544280^3 + 1294899176535720^3 = 727959282778000^3 + 1224915311765600^3 = 857010857812200^3 + 1168192425418200^3 = 1009237516560000^3 + 1061381454915600^3.

After a(1) = 3 this is always composite, because factorization of the polynomial a^3 + b^3 into irreducible components over Z is a^3 + b^3 = (b+a)*(b^2 - ab + b^2). They may be semiprimes, as with 671 = 11 * 61, and 1261 = 13 * 97. The numbers can be powers in various ways, as with 32 = 2^5, 81 = 3^4, 256 = 2^8, 784 = 2^4 * 7^2 , 1225 = 5^2 * 7^2, and 2187 = 3^7. - Jonathan Vos Post, Feb 05 2011

If n is a term then n*b^3 is also a term for any b, e.g., 3 is a term hence 3*2^3 = 24, 3*3^3 = 81 and also 3*4^3 = 192 are terms. Sequence of primitive terms may be of interest. - Zak Seidov, Dec 11 2013

First noncubefree primitive term is 168 = 21*2^3 (21 is not a term of the sequence). - Zak Seidov, Dec 16 2013

REFERENCES

Ian Stewart, "Game, Set and Math", Chapter 8, 'Close Encounters of the Fermat Kind', Penguin Books, Ed. 1991, pp. 107-124.

LINKS

T. D. Noe and Harry J. Smith, Table of n, a(n) for n = 1..1000

FORMULA

a(n) = sqrt(A050802(n)). - Jonathan Sondow, Oct 28 2013

EXAMPLE

E.g. 1183^2 = 65^3 + 104^3.

MATHEMATICA

Select[Range[5350], Reduce[0 < x <= y && #^2 == x^3 + y^3, {x, y}, Integers] =!= False &] (* Jean-Fran├žois Alcover, Mar 30 2011 *)

Sqrt[#]&/@Union[Select[Total/@(Tuples[Range[500], 2]^3), IntegerQ[ Sqrt[ #]]&]] (* Harvey P. Dale, Mar 06 2012 *)

PROG

(PARI) nstart=1; astart=3; n=nstart; a=astart-1; until (0, a++; a2=a^2; b1=((a2/2)^(1/3))\1; for (b=b1, a, b3=b^3; c1=1; if (a2 > b3, c1=((a2-b3)^(1/3))\1; ); for (c=c1, b, d=b3 + c^3; if (d > a2 && c == 1, break(2)); if (d > a2, break); if (a2 == d, print(n, " ", a); write("b050801.txt", n, " ", a); n++; break(2))))) \\ Harry J. Smith, Jan 15 2009

(PARI) is(n)=my(N=n^2); for(k=sqrtnint(N\2, 3), sqrtnint(N-1, 3), if(ispower(N-k^3, 3), return(n>1))); 0 \\ Charles R Greathouse IV, Dec 13 2013

CROSSREFS

Cf. A050802, A000404, A033430, A001105, A038597, A050803, A106265, A217248.

Sequence in context: A032831 A047180 A051394 * A103093 A124632 A048091

Adjacent sequences:  A050798 A050799 A050800 * A050802 A050803 A050804

KEYWORD

nonn,nice

AUTHOR

Patrick De Geest, Sep 15 1999.

EXTENSIONS

More terms from Michel ten Voorde (seqfan(AT)tenvoorde.org) and Jud McCranie.

STATUS

approved

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Last modified April 19 05:22 EDT 2014. Contains 240738 sequences.