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  1. ; recognizing fibonacci numbers
  2.  
  3.   (define (square? n)
  4.     (define (isqrt n)
  5.       (if (not (and (positive? n) (integer? n)))
  6.           (error 'isqrt "must be positive integer")
  7.           (let loop ((x n))
  8.             (let ((y (quotient (+ x (quotient n x)) 2)))
  9.               (if (< y x) (loop y) x)))))
  10.     (let ((x (isqrt n)))
  11.       (= (* x x) n)))
  12.  
  13. (define (fibonacci? x)
  14.   (if (< x 1) #f
  15.     (let loop ((prev 1) (fib 1))
  16.       (if (< fib x) (loop fib (+ fib prev))
  17.         (= fib x)))))
  18.  
  19. (display (fibonacci? 13)) (newline)
  20. (display (fibonacci? 15)) (newline)
  21.  
  22. (define (fibonacci? x)
  23.   (let ((5xx (* 5 x x)))
  24.     (or (square? (+ 5xx 4))
  25.         (square? (- 5xx 4)))))
  26.  
  27. (display (fibonacci? 13)) (newline)
  28. (display (fibonacci? 15)) (newline)
Success #stdin #stdout 0.02s 8344KB
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#t
#f
#t
#f
https://ideone.com/XqUP3W
language:
Scheme (guile 2.2.4)
created:
6 years ago
visibility:
public

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