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 Register FAQ Search Today's Posts Mark Forums Read 2020-03-21, 21:47 #1 enzocreti   Mar 2018 49610 Posts Diophantine equation Are there infinitely many solutions to these Diophantine equation 10^n-a^3-b^3=c^2 with n, a, b, c positive integers?   2020-03-21, 21:51 #2 R. Gerbicz   "Robert Gerbicz" Oct 2005 Hungary 17×79 Posts Yes.   2020-03-21, 21:54   #3
enzocreti

Mar 2018

24·31 Posts Quote:
 Originally Posted by R. Gerbicz Yes.
How to proof it?   2020-03-21, 23:00 #4 VBCurtis   "Curtis" Feb 2005 Riverside, CA 101348 Posts Take an algebra class. Learn how to answer your own questions. Even wikipedia articles would give you sufficient tools to answer your curiosities.   2020-03-22, 04:46   #5
CRGreathouse

Aug 2006

10110111001002 Posts Quote:
 Originally Posted by enzocreti Are there infinitely many solutions to these Diophantine equation 10^n-a^3-b^3=c^2 with n, a, b, c positive integers?
It seems rather difficult to find positive integers n such that there are no positive integers a, b, and c with 10^n = a^3 + b^3 + c^2. n = 5 is an example, but there are no others up to 18.

I don't know of any modular obstructions.   2020-03-22, 08:39   #6
R. Gerbicz

"Robert Gerbicz"
Oct 2005
Hungary

17·79 Posts Quote:
 Originally Posted by CRGreathouse It seems rather difficult to find positive integers n such that there are no positive integers a, b, and c with 10^n = a^3 + b^3 + c^2. n = 5 is an example, but there are no others up to 18. I don't know of any modular obstructions.
Multiple the equation by 10^(6*k), since it is cube and square if there is a solution for n=N, then there is a solution for N+6*k.   2020-03-23, 03:03   #7
CRGreathouse

Aug 2006

22·5·293 Posts Quote:
 Originally Posted by R. Gerbicz Multiple the equation by 10^(6*k), since it is cube and square if there is a solution for n=N, then there is a solution for N+6*k.
Ah! of course. So it suffices to show
10^1 = 1^3 + 2^3 + 1^2
10^2 = 3^3 + 4^3 + 3^2
10^3 = 6^3 + 7^3 + 21^2
10^4 = 4^3 + 15^3 + 81^2
10^6 = 7^3 + 26^3 + 991^2
10^11 = 234^3 + 418^3 + 316092^2
and then we know that all powers of 10, other than 10^5, are expressible as the sum of two positive cubes and a positive square.  Thread Tools Show Printable Version Email this Page Similar Threads Thread Thread Starter Forum Replies Last Post Nick Number Theory Discussion Group 2 2016-12-18 14:49 flouran Math 7 2009-12-12 18:48 grandpascorpion Math 11 2009-09-23 03:30 Batalov Puzzles 3 2009-04-10 11:32 philmoore Puzzles 8 2008-05-19 14:17

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