FriCAS Code

30 lines (23 with data), 746 Bytes

--Copyright The Numerical Algorithms Group Limited 1991.

--  This is the 3n + 1 function:   f(n) = n/2    if n is even,
--                                        3n + 1 otherwise
--
-- It can obviously be coded as
--  f n ==
--    even? n => n quo 2
--    3 * n + 1

-- but we code using the pattern matching syntax:
-- f n ==
--    n is 2*m% => m%
--    3 * n + 1

-- The 'integer' is to return an integer instead of a polynomial
f n ==
   not empty?(u := Is(n, 2*m%)) => integer eval(m%, u)
   3 * n + 1

)set stream showall on

-- The function g generates the stream of the repeated applications of f
g(n:INT):STREAM(INT) == stream(f, n)

-- There is a conjecture that g(n) gets into the cycle (1 4 2)
-- for every n
s := g 27
extend(s, 150)