f[0]=af[0]=af[0]=a(反正是个常数)
f[i]=∑j=1if[i mod j]f[i]=\sum_{j=1}^{i}f[i\bmod j]f[i]=∑j=1if[imodj]
求 f[n],n≤5×104f[n],n\le5 \times 10^4f[n],n≤5×104