Python 3.5, 69 bytes, Cracked in comments

def f(x):
 y=str(x)
 while not(x and(y[-1]+y[:-1])==str(2*x)):a=1

There is a solution that is an integer.

Cracked by isaacg (in 11 min) and Jarko Dubbeldam.

Python 3.5, 69 bytes, Cracked

def f(x):
 y=str(x)
 while not(x and(y[-1]+y[:-1])==str(2*x)):a=1

There is a solution that is an integer.

Cracked by isaacg (in 11 min) and Jarko Dubbeldam.

Python 3.5, 69 bytes##

def f(x):
 y=str(x)
 while not(x and(y[-1]+y[:-1])==str(2*x)):a=1

There is a solution that is an integer.

Python 3.5, 69 bytes, Cracked in comments

def f(x):
 y=str(x)
 while not(x and(y[-1]+y[:-1])==str(2*x)):a=1

There is a solution that is an integer.

Cracked by isaacg (in 11 min) and Jarko Dubbeldam.