Are there any whole numbers that satisfy this equation?

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In summary, there are whole numbers for x, y, a, b, that satisfy x^3-y^2+ab=a-b(x+y). However, it is disputed whether 0 is a whole number.
  • #36
Yes. Put ## (a+x+y) ## on the left side. Since the OP is not responding, and @archaic has sort of taken over the function of the OP, let me show you what I got, and see if you agree. I get
## (a+x+y)(b-1)=y(y-1)-x(x^2+1) ##.## \\ ## Suggestion is to let ##b=2 ##, ##x=1 ##, let ##y ## take on all kinds of values, and solve for ## a ##. This I think is what @haruspex has been looking for, and it is a very good way to get a bunch of solutions for this problem. Hopefully my algebra is accurate.
 
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  • #37
Charles Link said:
Suggestion is to let b=2 , x=1 , let y take on all kinds of values, and solve for a .
Yes, that gives an infinite family of solutions. But there are plenty of others to be had by taking any value of x, then a sufficiently large value of y to make the RHS greater than x+y, then factorise that however you can.
 
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  • #38
Charles Link said:
Yes. Put ## (a+x+y) ## on the left side. Since the OP is not responding, and @archaic has sort of taken over the function of the OP, let me show you what I got, and see if you agree. I get
## (a+x+y)(b-1)=y(y-1)-x(x^2+1) ##.## \\ ## Suggestion is to let ##b=2 ##, ##x=1 ##, let ##y ## take on all kinds of values, and solve for ## a ##. This I think is what @haruspex has been looking for, and it is a very good way to get a bunch of solutions for this problem. Hopefully my algebra is accurate.
I was thinking of ##b=2## too, but I didn't go back to the equation so I thought it was ##(b-1)##, not its opposite, and that for that ##b## I'd see ##x+y## cancel from the RHS and LHS, but it was ##(1-b)## :/
 
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  • #39
archaic said:
I was thinking of ##b=2## too, but I didn't go back to the equation so I thought it was ##(b-1)##, not its opposite, and that for that ##b## I'd see ##x+y## cancel from the RHS and LHS, but it was ##(1-b)## :/
To illustrate post #36, arbitrarily set x=5. That means y must be chosen to be at least 13, but that would only allow the b=2 solution we already have. So let's make y a bit larger, 15 say:
(a+20)(b-1)=80
We can factorise 80 in any way that makes one factor at least 21.. so 40x2, allowing a=20, b=3.
 
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  • #40
haruspex said:
But there are plenty of others to be had by taking any value of x, then a sufficiently large value of y to make the RHS greater than x+y, then factorise that however you can.
##(a+x+y)(b-1)=-x^3+y^2-x-y##
I am not sure why you want the RHS to be greater than ##x+y##. In the case of ##b=2##, we can have ##a## to be negative in order to bring the sum back to the RHS. For ##b## different than ##2## I don't see why you want that.
 
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  • #41
archaic said:
##(a+x+y)(b-1)=-x^3+y^2-x-y##
I am not sure why you want the RHS to be greater than ##x+y##. In the case of ##b=2##, we can have ##a## to be negative in order to bring the sum back to the RHS. For ##b## different than ##2## I don't see why you want that.
As clarified in post #14, all numbers have to be positive. The minimum of the LHS is x+y+1.
 
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  • #42
haruspex said:
As clarified in post #14, all numbers have to be positive. The minimum of the LHS is x+y+1.
Perhaps, it can be that ##b = 1##. Then the LHS is zero.
 
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  • #43
SammyS said:
Perhaps, it can be that b=1b=1b = 1. Then the LHS is zero.
See my post 23. Before doing it by @haruspex 's method, I did find a case where ## b=1 ##, ## x=1 ## and ## y=2 ##. ## \\ ## I see that ## x=3 ## and ##y=6 ## also works. If the whole numbers include zero, ## x=0 ## and ## y= 1 ## also works. ## \\ ## Looks like ## a ## can be arbitrary=anything for these=it's obvious also by looking at the original expression in post 1.
 
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