Compilare: user.cpp: In function ‘char recursiv(long long unsigned int)’: user.cpp:22:11: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] if(n<=k) ^ user.cpp: In function ‘int main()’: user.cpp:134:9: warning: unused variable ‘u’ [-Wunused-variable] ull u=2; ^ user.cpp: In function ‘char recursiv(long long unsigned int)’: user.cpp:103:1: warning: control reaches end of non-void function [-Wreturn-type] } ^