ppp是个质数，关于a,b,c,d∈(0,p)a,b,c,d∈(0,\sqrt{p})a,b,c,d∈(0,p)的不定方程ad+bc=pad+bc=pad+bc=p的正整数解有f(p)f(p)f(p)个，怎么证明
f(p)=4(∑m<pφ(m))−p−1f(p)=4(\sum\limits_{m<\sqrt{p}}φ(m))-p-1f(p)=4(m<p∑φ(m))−p−1
