restart: n := 6: # Change to whatever you wish
 # g(n) calcutates the number-theoretic function
 # considered by Collatz

 g := proc(n:posint)
 local m;
 m := modp(n,3);
 if m = 0 then 2*n/3;
 elif m=1 then (4*n-1)/3;
 else (4*n+1)/3;
 fi;
 end:


 print(`g(` .n. `)` = g(n));