Prime numbers.

Prime numbers are divisible only by 1 and by themselves. They stand in their place in the infinite series of natural numbers, squashed in between two others, like all other numbers, but a step further on than the rest. They are suspicious and solitary, which is why Mattia thought they were wonderful. Sometimes he thought that they had ended up in that sequence by mistake, that they’d been trapped like pearls strung on a necklace. At other times he suspected that they too would rather have been like all the others, just ordinary numbers, but for some reason they weren’t capable of it.

The Solitude of Prime Numbers, Paul Giordano.