When is a rational number a natural number?

Answer 1

A rational number in ratio from is a natural number if

• the numerator is 0 or both its numerator and denominator have the same sign, and
  
• it is equivalent to an integer ratio with denominator 1.

A rational number in decimal form is a natural number if

• it is non-negative, and
• the decimal point is followed by only 0s or followed by an infinite string of 9s.

Answer 2

When it's a positive integer.