I saw that definition of divide and conquer in various places including wikipedia’s master theorem explanation, MIT6046 lecture, etc.
But I don’t understand how can the number “a” equal to 1, i.e., if I divide one problem into one subproblem, aren’t they a same problem?
Also, why isn’t the size for each subproblem n/a, but n/b where b is a positive real number larger 1?

Thanks in advance.