q=1时,Sn/S2n=(n*a1)/(2n*a1)=1/2q≠1,Sn/S2n=(1-q^n)/(1-q^2n)=1/(1+q^n)当|q|＜1时,lim Sn/S2n=lim1/(1+q^n)=1当|q|＞1时,lim Sn/S2n=lim1/(1+q^n)=0所以P={0,1,1/2}.其子集有8个