# Tag Archives: math

## “Presto” math trick in Java

According to Futility Closet, if you start with a 3 digit and place it next to the same number to form a 6 digit number, you can divide the 6 digit number by 7, 11, and then 13 and you will end up with the original 3 digit number and no remainders. For example, by taking the number 412 and making it 412412 and then doing the divisions, you will end up with 412. I wrote a small program in Java to test it.

## Recursive Collatz conjecture in PHP

I stumbled across a blog post by Brie Gordon about the Collatz conjecture, so naturally I had to Wikipedia it. The simple story is that for any given integer, if it is even you divide it by 2 and if it is odd you multiply it by 3 and add 1. You continue this pattern with the returned numbers, and eventually the number will reach 1. So, to kill some time, I wrote a recursive version in PHP.  Here it is:

```function collatz(\$num)
{
if(\$num == 1)
{
echo "<p style='color: red'>" . \$num . "</p>\n";
return;
}

if(\$num % 2 == 0) //even
{
echo "<p style='color: green'>" . \$num . "</p>\n";
return collatz(\$num / 2);
}
else
{
echo "<p style='color: red'>" . \$num . "</p>\n";
return collatz(3 * \$num + 1);
}
}```