OFFSET
0,1
COMMENTS
The 3x + 1 problem remains unsolved. But the -3x + 1 problem is easy to answer definitively: none of the thirty-one numbers in this sequence reach 1 because they form a cycle that does not include 1.
FORMULA
a(0) = 13; a(n) = (-3)*a(n - 1) + 1 if a(n - 1) is odd, a(n) = a(n - 1)/2 otherwise.
EXAMPLE
13 is odd, so it's followed by (-3) * 13 + 1 = -39 + 1 = -38.
-38 is even, so it's followed by -38/2 = -19.
MATHEMATICA
NestList[If[EvenQ[#], #/2, -3# + 1] &, 13, 100]
PROG
(Magma) [n eq 1 select 13 else IsOdd(Self(n-1)) select -3*Self(n-1)+1 else Self(n-1) div 2: n in [1..80]]; // Vincenzo Librandi, Jun 11 2018
CROSSREFS
KEYWORD
sign,easy
AUTHOR
Alonso del Arte, May 26 2018
STATUS
approved