Question

• Is Hilbert's tenth problem for Diophantine equations of power $3$ in rational numbers decidable?
• Is Hilbert's tenth problem for Diophantine equations in rational numbers of power $3$ decidable?
• Is there a universal Diophantine equation of power $3$?
• Is there a universal Diophantine equation containing less than $9$ variables? If so, what is the minimal number of variables? What minimal power can be achieved for that number of variables?
• Is there a universal Diophantine equation that can be written using less than $100$ arithmetic operations (additions or multiplications)? If so, what is the minimal number of operations?