Which of these is an example of a recursive function?

def FunctionOne$($N$)$:

if N $<=0$:

return $0$

else:

return pow$(2,$ N$-1)$

def FunctionTwo$($N$)$:

result $=1$

for i in range$($N$)$:

result $=$ result $*2$

return result

def FunctionThree$($N$)$:

return max$($N$,$ max$($N$,0))$

def FunctionFour$($N$)$:

if N $<=1$:

return $1$

return $2*$FunctionFour$($N$-1)$