This paper shows that the equation in the title has no solutions in the positive integers if (i) n has prime factor congruent to 3 (modulo 4) ; or (ii) 8|n; or 8|n-1. Case (i) follows from an elementary argument. Case (ii) follows from the previous works of E. Dofs and the second author. Case (iii) follows from a standard Brauer-Manin obstruction argument.
Tân et al. (Mon,) studied this question.
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