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I have thought of a single simple rule for Colatz conjecture that is slightly different to the normal one. Here it is:
$$f:(a,b)\rightarrow (3a+\gcd(a,b),b)$$
Then the Colatz conjecture can be written as:
$$\forall_{n:\mathbb{N}}\exists_{m:\mathbb{N}}\exists_{p:\mathbb{N}}\{ f^p(n,2^m) = (2^m,2^m...