But what happens if we change the rules of the sequence, and divide by two for even numbers, but multiply by FIVE and add one for odd numbers? So instead of 3n+1 the next term is 5n+1. My searching on the internet has not identified anybody who has previously followed this line of research, so I have decided to pursue it.
A variation on the Collatz Conjecture: 5n+1
For the Collatz conjecture, all series reach one (or so it seems, there's no formal proof yet). However, if we follow a new algorithm, and multiply by five and add one for the odd numbers, we enter one of a number of different scenarios (I've identified four,but this is not conclusive or exhaustive).
Firstly, trying this variation of the Collatz series, starting with 1. Seems like a good place to start, and it's not as trivial as the original series. Starting with 1 produces a loop:
1 -- 6 -- 3 -- 16 -- 8 -- 4 -- 2 -- 1 which is the simplest loop, "Loop A" and contains 1, 2, 3 and 4.
Secondly: (5 -- 26 --) 13 -- 66 -- 33 -- 166 -- 83 -- 416 -- 208 -- 104 -- 52 -- 26 -- 13 which is another loop, "Loop B"
So far, this covers 1, 2, 3, 4, 5 and 6, 8, and 10 (which divides by two to enter Loop B).
Thirdly: However, something that I found strange and unexpected happens if we start with 11 (which goes through 7 after four steps)
* The series does not appear to enter a loop, it just keeps growing and growing - although I can't prove it.
* The odd numbers that series includes are either prime - shown in red - or semi-prime (only having prime factors), shown in blue.
* The first non-prime in the series is 9, which is the square of a prime (3).
The fourth variation I've seen starts with 17. It seems that there's no pattern for other odd numbers which have not been in the previous loops, so I'm working through them. For example, 15 enters the growing sequence at 46 (in the top row of the diagram of the sequence shown above), but 17 enters a different loop, Loop C.
The 5n+1 variation of the Collatz conjecture leads to one of four situations:
Loop A: 1 - 6 - 3 - 16 - 8 - 4 - 2 - 1
Loop B: (5 - 26 -) 13 - 66 - 33 - 166 - 83 - 416 - 208 - 104 - 52 - 26 - 13
Loop C: 17 - 86 - 43 - 216 - 108 - 54 - 27 - 136 - 68 - 34 - 17
Sequence: Starting with 11 (and including 7) - limitless growth (it does not appear to enter a loop)
* To look for additional loops, continuing to review the prime numbers not already covered.
* To try another variation on the Collatz Conjecture, for example, 5n-1 instead of 5n+1. Clearly, 5n+1 has the scope to grow very rapidly with almost no chance of decreasing, and the only observations that can be made are on the numbers that the sequence goes through.