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Published online by Cambridge University Press: 28 June 2013
We investigate sums of mixed powers involving two squares and three biquadrates. In particular, subject to the truth of the Generalised Riemann Hypothesis and the Elliott–Halberstam conjecture, we show that all large natural numbers
$n$ with
$8\nmid n$,
$n\not\equiv 2~(\text{mod} ~3)$ and
$n\not\equiv 14~(\text{mod} ~16)$ are the sum of two squares and three biquadrates.