archaic
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you mean this ##(a-x-y)(b+1)=y(y+1)-x(x-1)(x+1)## ?haruspex said:Neither.
Get it into the form (a+...)(b+..)=...
you mean this ##(a-x-y)(b+1)=y(y+1)-x(x-1)(x+1)## ?haruspex said:Neither.
Get it into the form (a+...)(b+..)=...
That's the idea, but you seem to have made some sign errors.archaic said:you mean this ##(a-x-y)(b+1)=y(y+1)-x(x-1)(x+1)## ?
$$x^3-y^2+ab=a-b(x+y)\\x^3-y^2=a-ab-b(x+y)$$haruspex said:That's the idea, but you seem to have made some sign errors.
When you've fixed those, consider what order of picking values is likely to be helpful. If you plug in arbitrary values for a, b and one of x, y, you will have a quadratic or cubic to solve. What would be a better procedure?
a=-x-y is no use since all have to be positive, and b=1 is of limited value since it still leaves you with an awkward equation in x and y to solve.archaic said:Two arrangements that stand out: when ##b=1## or ##a=-x-y##.
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.Charles Link said:Suggestion is to let b=2 , x=1 , let y take on all kinds of values, and solve for a .
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)## :/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.
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: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)## :/
##(a+x+y)(b-1)=-x^3+y^2-x-y##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.
As clarified in post #14, all numbers have to be positive. The minimum of the LHS is x+y+1.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.
Perhaps, it can be that ##b = 1##. Then the LHS is zero.haruspex said:As clarified in post #14, all numbers have to be positive. The minimum of the LHS is x+y+1.
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.SammyS said:Perhaps, it can be that b=1b=1b = 1. Then the LHS is zero.