What is the equivalent of unsigned long (c++) ul in python.
Unless you're relying on the limitations of unsigned long (e.g. rollover from ULONG_MAX to 0 on increment and vice-versa on decrement; inability to store negative values), you don't need to worry. Python's int type is "infinite precision" (it has limits, because real world computers can't do infinite, but the limits are high enough that you will never encounter them unless you try to do stuff like RSA crypto math without using three-arg pow, and even then, it's typically just slow, it still works). So for your use case:
a = math.sqrt(1 + (4 * x)) # Or (1 + (4 * x)) ** 0.5
works just fine.
If you need to rely on the limitations of unsigned long, it's uglier; you can use explicit masking with 0xFFFF_FFFF_FFFF_FFFF after every operation that might overflow, or using the ctypes.c_ulong type for all your math (with constant casts back to c_ulong), but that's not typically needed, and not needed for your use case.