Prime number sieve using LCM function

sieve.py

# Author: Pedja 



print("                        ***** SIEVE *****\n\n\n")
while True:
	
    
   
    n1=int(input("Enter lower bound : "))
    n2=int(input("Enter upper bound : "))
	
    def lcm(p,q):
        p, q = abs(p), abs(q)
        m = p * q
        if not m: return 0
        while True:
            p %= q
            if not p: return m // q
            q %= p
            if not q: return m // p
    def sieve(n1,n2):
        b1=2
        b2=2
        x=2
        k=3
        i=1
        while x<=n2:
            if i==1 and x>=n1:
                print(int(x))
                i=0
            b3=b1+lcm(k-1,b1)
            a=b3/b1-1
            k=k+1
            b1=b2
            b2=b3
            if x<a:
                x=a
                i=1
        return ""
        
	



   
    if n1<0:
        print("Lower bound must be nonnegative number")
    elif n1>n2:
        print("Upper bound mustn't be less than lower bound")
    else:
        print(sieve(n1,n2))
    
		
    try_again = ""
    # Loop until users opts to go again or quit
    while not(try_again == "1") and not(try_again == "0"):
        try_again = input("Press 1 to try again, 0 to exit. ")
        if try_again in ["1", "0"]:
            continue  # a valid entry found
        else:
            print("Invalid input- Press 1 to try again, 0 to exit.") 
    # at this point, try_again must be "0" or "1"
    if try_again == "0":
        break