Doug Orleans's Journal

FWIW, here's my brute-force solution. I screwed up initially with (- s n) instead of (- n s).

(define (problem-46)
(let loop ((k 3))
(if (or (prime? k)
(sum-of-a-prime-and-twice-a-square? k))
(loop (+ k 2))
k)))

(define (sum-of-a-prime-and-twice-a-square? n)
(any prime? (map (lambda (s) (- n s))
(twice-squares-less-than n))))

(define (twice-squares-less-than n)
(let loop ((k 1) (results '()))
(let ((s (* 2 k k)))
(if (< s n)
(loop (+ k 1) (cons s results))
results))))

(define (prime? n)
(or (= n 2)
(and (not (even? n))
(let testing ((k 3))
(or (< n (* k k))
(and (not (= 0 (remainder n k)))
(testing (+ k 2))))))))