In this poster, we give two recurrence sequences of integers, defined by recurrence relations of the form V_{n+1}=f(V_n;T_n) and T_{n+1}=g(V_n;T_n), which are related to the Collatz conjecture. We show why if we can prove that lim_{{n → +∞}} V_n=0 for all the values (V₀;T₀) that means Collatz’s conjecture proof. We also try to understand why the Collatz conjecture is a difficult problem.