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Ω: lower bound. A function g is in Ω (f) iff there are positive constants c and n0 such that g(n) ≥ cf(n) for all n ≥ n0.
Θ: tight bound. A function g is in Θ (f) iff g is in O(f) and g is in Ω(f).
Why are these operators useful for understanding the cost of evaluating procedures?
(define (sort lst cf) (if (null? lst) lst (let ((best (find-best lst cf))) (cons best (sort (delete lst best) cf))))) (define (find-best lst cf) (if (= 1 (length lst)) (car lst) (pick-better cf (car lst) (find-best (cdr lst) cf))))
(define (insertsort cf lst) (if (null? lst) null (insertone cf (car lst) (insertsort cf (cdr lst))))) (define (insertone cf el lst) (if (null? lst) (list el) (if (cf el (car lst)) (cons el lst) (cons (car lst) (insertone cf el (cdr lst))))))