13. 已知f(x)f(x)f(x)为奇函数,当x<0x<0x<0时,f(x)=x2−2x−1f(x)=x^{2}-2x-1f(x)=x2−2x−1;则当x>0x>0x>0,f(x)f(x)f(x)的解析式为f(x)=f(x)=f(x)=___−x2−2x+1-x^{2}-2x+1−x2−2x+1___.