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%I #22 Sep 08 2022 08:46:12
%S 79,238,119,358,179,538,269,808,404,202,101,304,152,76,38,19,58,29,88,
%T 44,22,11,34,17,52,26,13,40,20,10,5,16,8,4,2,1,4,2,1,4,2,1,4,2,1,4,2,
%U 1,4,2,1,4,2,1,4,2,1,4,2,1,4,2,1,4,2,1,4,2,1,4,2,1,4,2,1,4,2,1,4,2,1,4
%N 3x + 1 sequence starting at 79.
%F a(0) = 79; a(n) = 3*a(n - 1) + 1 if a(n - 1) is odd, a(n) = a(n - 1)/2 otherwise.
%e 79 is odd, so it's followed by 3 * 79 + 1 = 238.
%e 238 is even, so it's followed by 238/2 = 119.
%t NestList[If[EvenQ[#], #/2, 3# + 1] &, 79, 100]
%o (Scheme, with memoization-macro definec)
%o (definec (A258098 n) (if (zero? n) 79 (A006370 (A258098 (- n 1)))))
%o (define (A006370 n) (if (even? n) (/ n 2) (+ 1 n n n))) ;; Image of n under the `3x+1' map.
%o ;; _Antti Karttunen_, May 26 2015
%o (Magma) [n eq 1 select 79 else IsOdd(Self(n-1)) select 3*Self(n-1)+1 else Self(n-1) div 2: n in [1..100]]; // _Vincenzo Librandi_, May 27 2015
%o (Scala) def collatz(n: Int): Int = n % 2 match {
%o case 0 => n / 2
%o case _ => 3 * n + 1
%o }
%o def collatzSeq(n: Int): LazyList[Int] = LazyList.iterate(n)(collatz)
%o collatzSeq(79).take(100).toList // _Alonso del Arte_, Apr 26 2020
%Y Cf. A006370, A033478, A008874, A258056.
%Y Row 79 of A347270.
%K nonn,easy
%O 0,1
%A _Alonso del Arte_, May 19 2015
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