Problem 1

A technologically advanced colony of army ants sets out to infiltrate colonies of the rival leaf cutter ants using ant-droids. They already manufactured 250000 ant-droids, each identified by their unique production number between 1 and 250000. However, upon switching them on, the army ants realized that due to an unfortunate programming error, any ant-droid attacked the ant-droids whose identifiers were multiples of their own identifier. The queen ordered the ant-droid army to be divided into smaller units in which no ant-droid wants to attack an other. At least how many units need to be created?