By elementary compute, we can easily get an asymptotic formula for the number of solutions for the equation n\_1+n\_2+n\_3=N. Here N is a fixed positive integer, sufficiently large. My question is: can we apply circle method to this equation and get a same asymptocic formula for the number of solutions?



A naive question on circle method
I don't think the circle method works well this case. The method is based on the idea that the generating function has local peaks at rational numbers with sufficiently rapid local decay there (in particular it is small at points which are not close to a rational number with small denominator). In our case the generating function is the cube of the geometric series $\sum\_{n=0}^N e(n\alpha)$ whose only peak is at $\alpha=0$, namely it is rather small when $\|\alpha\|$ is considerably larger than $1/N$. In other words, in our case only one rational number (namely $\alpha=0$) has a real impact at the integral, but as a by-product the decay is not as definite as in higher degree cases: the transitional range $\|\alpha\|\asymp 1/N$ is rather subtle. To put differently, in this case the mass in the integral is very unevenly distributed, hence it seems as if one would need to dissect the circle in a completely trivial and useless way: there would be one major arc (the whole circle) and no minor arc at all.


**EDIT.** It is instructive to look at the treatment of Waring's problem in Vaughan's book "The Hardy-Littlewood method", and see "what goes wrong" when $k=1$. Well, nothing goes wrong but it turns out that the method itself furnishes no information in this case. Indeed, the singular integral given by (2.9) and (2.15) is approximated by the full integral (2.20) which is really the number of solutions to $n\_1+\cdots+n\_s=N$ when $k=1$. This is then evaluated (for any $k$) in an elementary fashion in (2.22) of the book.


