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Task/Stern-Brocot-sequence/EchoLisp/stern-brocot-sequence-2.echolisp

@@ -0,0 +1,20 @@
+; generate the sequence and check GCD
+(for ((n 10000))
+    (unless (= (gcd (stern n) (stern (1+ n))) 1) (error "BAD GCD" n)))
+    → #t
+
+;; first items
+(define sterns (cache 'stern))
+(subvector sterns 1 16)
+   → #( 1 1 2 1 3 2 3 1 4 3 5 2 5 3 4)
+
+;; first occurences index
+(for ((i (in-range 1 11))) (write (vector-index i sterns)))
+→  0 3 5 9 11 33 19 21 35 39
+
+;; 100
+(writeln (vector-index 100 sterns))
+→ 1179
+
+(stern 900000) → 446
+(stern 900001) → 2479