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Solve in Integers (Posted on 2023-10-01) Difficulty: 3 of 5
Find all possible integer solutions that satisfy this system of equations:
a+b+c=1
a3+b3+c2=1

See The Solution Submitted by K Sengupta    
Rating: 5.0000 (1 votes)

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Some Thoughts Probably all, though not proved | Comment 4 of 5 |
There is an infinite number of solutions of the form (k, -k, 1).

Algebraic manipulation shows 4 more solutions:
c = 1-a-b
c^2 = 1 + a^2 + b^2 + 2(ab - a - b)
a^3 + b^3 + c^2 = a^3+b^3+ 1 + a^2 + b^2 + 2(ab - a - b) = 1
a^3 + b^3 + a^2 + b^2 + 2(ab - a - b) = 0
(a^3 + a^2 - 2a) + (b^3 + b^2 - 2b) + 2ab = 0
a(a+2)(a-1) + b(b+2)(b-1) + 2ab = 0

By inspection if either a or b is zero, then the other (b or a) can be {0, 1, -2} and the equation is satisfied giving 6 solutions, though 2 of them are both (0,0,1) and are included in the infinite set noted above.  The other four:
(0,1,0), (0,-2,3), (1,0,0), (-2,0,3).

There are 2 others that I didn't find until running the program.  I suspect further algebraic manipulation would find them.  [Edit: it looks like broll already did so]

The complete set:
    (a,b,c)
    (k, -k, 1) for all integers k
    (-3,-2,6)
    (-2,-3,6)
    (-2,0,3)
    (0,-2,3)
    (0,1,0)
    (1,0,0)

----------------------
big = 1000
for a in range(-big,big+1):
    for b in range(-big,big+1):
        if a == -b:
            continue
        c = 1-a-b
        if a**3 + b**3 + c**2 == 1:
            print('({},{},{})'.format(a,b,c))

Edited on October 1, 2023, 8:46 am
  Posted by Larry on 2023-10-01 08:45:09

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