Lander, Parkin, and Selfridge conjecture
The Lander, Parkin, and Selfridge conjecture concerns the integer solutions of equations which contain sums of like powers. The equations are generalisations of those considered in Fermat's Last Theorem. The conjecture is that if the sum of some "k"-th powers equals the sum of some other "k"-th powers, then the total number of terms in both sums combined must be at least "k". Background. Diophantine equations, such as the integer version of the equation "a" + "b" = "c" that appears in the Pythagorean theorem, have been studied for their integer solution properties for  centuries. Fermat's Last Theorem states that for powers greater than 2, the equation "a" + "b" = "c" has no solutions in non-zero integers "a", "b", "c". Extending the number of terms on either or both sides, and allowing for higher powers than 2, led to Leonhard Euler to propose in 1769 that for all integers "n" and "k" greater than 1, if the sum of "n" "k"th powers of positive integers is itself a "k"th power, then "n" is greater than or equal to "k". In symbols, if formula_1 where "n" > 1 and formula_2 are positive  integers, then his conjecture was that "n" ≥ "k". In 1966, a counterexample to Euler's sum of powers conjecture was found by Leon J. Lander and Thomas R. Parkin for "k" = 5: In subsequent years, further counterexamples were found, including for "k" = 4. The latter disproved the more specific Euler quartic conjecture, namely that "a" + "b" + "c" = "d" has no positive integer solutions. In fact, the smallest solution, found in 1988, is Conjecture. In 1967, L. J. Lander, T. R. Parkin, and John Selfridge conjectured that if formula_3, where "a" ≠ "b" are positive  integers for all 1 ≤ "i" ≤ "n" and 1 ≤ "j" ≤ "m", then "m"+"n" ≥ "k". The equal sum of like powers formula is often abbreviated as (k, "m", "n"). Small examples with formula_4 (related to generalized taxicab number) include formula_5 (known to Euler) and formula_6 (found by K. Subba Rao in 1934). The conjecture implies in the special case of "m" = 1 that if (under the conditions given above) then "n" ≥ "k" − 1. For this special case of "m" = 1, some of the known solutions satisfying the proposed constraint with " n" ≤ "k , where terms are positive integers, hence giving a partition of a power into like powers, are: Fermat's Last Theorem states that for k" = 4 the conjecture is true. Current status. It is not known if the conjecture is true, or if solutions exist that would be counterexamples, such as "a" + "b" = "c" + "d" for "k" ≥ 5.