Why does this pattern with consecutive numbers in the Collatz Conjecture work

collatz conjecturenumber theory

I have been getting into the Collatz Conjecture and I have noticed something interesting. Consecutive numbers often take the same amount of numbers to get to 1. For example, $84$ goes to $42$ to $21$ to $64$ to $32$ to $16$ to $8$ to $4$ to $2$ to $1$. That's ten numbers. $85$ goes to $256$ to $128$ to $64$ to $32$ to $16$ to $8$ to $4$ to $2$ to $1$. That's also ten numbers. There are many more examples, like $60$ and $61, 76$ and $77,$ and $92$ and $93.$ As the numbers get larger, the number of consecutive integers that take the same amount of numbers to get to one grows. $386, 387, 388, 389, 370,$ and $371$ all take $121$ numbers to get back to $1$! I have thought a lot about this, but I can't figure out why this pattern works. I also cannot find a pattern of when this pattern occurs. It seems to be random, but I might be wrong. Can someone help with these questions?

Best Answer

There are indeed patterns of this form.

If, say, you start with a number of the form $8n+4$ the chain begins

$$8n+4\mapsto 4n+2\mapsto 2n+1\mapsto 6n+4$$

While if you add $1$ to get $8n+5$ you get $$8n+5\mapsto 24n+16\mapsto 12n+8\mapsto 6n+4$$

Thus the consecutive numbers $8n+4,8n+5$ always have the same Collatz length. That explains your pairs $(84,85)$, $(60,61)$, $(76,77)$, $(92,93)$. I expect there are other patterns as well.