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Unexpected number of solutions from Solve with system of equations?

Posted 1 year ago

I want to solve a system of three equations to get the values of three variables. It doesn't matter whether I use Mathematica 13.1 or the Wolfram Engine 13.1, the results are the same. Here is the code:

Solve[{(a + 1)*(b + 1)*(c + 1) == 3 &&
           (a + 2)*(b + 2)*(c + 2) == -2 &&
           (a + 3)*(b + 3)*(c + 3) == -1},
           {a, b, c}, Reals] // N

I get six solutions:

{{a -> -3.1451, b -> -1.52398, c -> 1.66908}, {a -> -3.1451, 
  b -> 1.66908, c -> -1.52398}, {a -> -1.52398, b -> -3.1451, 
  c -> 1.66908}, {a -> -1.52398, b -> 1.66908, 
  c -> -3.1451}, {a -> 1.66908, b -> -3.1451, 
  c -> -1.52398}, {a -> 1.66908, b -> -1.52398, c -> -3.1451}}

Unfortunately, none of them is correct. And why do I get six solutions?

The correct answer is: a = 1.669079088, b = -1.523976396, c = -3.145102692

Do you have any suggestion how to improve the results?

POSTED BY: Jürgen Kanz
2 Replies

Your solution appears to agree with the last one provided by Solve. As for 6 solutions, fewer would make no sense unless there is multiplicity. There are three solutions and they are symmetric in the three variables. So any permutation of one solution will give another.

POSTED BY: Daniel Lichtblau

Okay, I understand. Thank you.

POSTED BY: Jürgen Kanz
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