Yellowstone sequence: Difference between revisions

<lang racket>#lang racket

(require plot)

(define a098550

(let ((hsh# (make-hash '((1 . 1) (2 . 2) (3 . 3))))

(rev# (make-hash '((1 . 1) (2 . 2) (3 . 3)))))

(λ (n)

(hash-ref hsh# n

(λ ()

(let ((a_n (for/first ((i (in-naturals 4))

#:unless (hash-has-key? rev# i)

#:when (and (= (gcd i (a098550 (- n 1))) 1)

(> (gcd i (a098550 (- n 2))) 1)))

i)))

(hash-set! hsh# n a_n)

(hash-set! rev# a_n n)

a_n))))))

(map a098550 (range 1 (add1 30)))

(plot (points

(map (λ (i) (vector i (a098550 i))) (range 1 (add1 100)))))</lang>

<pre>'(1 2 3 4 9 8 15 14 5 6 25 12 35 16 7 10 21 20 27 22 39 11 13 33 26 45 28 51 32 17)</pre>