665

665 is the natural number that follows 664 and precedes 666. It is an odd composite number. Its prime factorization is 5 × 7 × 19, so its positive divisors are 1, 5, 7, 19, 35, 95, 133, and 665.

In various numeral systems, 665 is written as 665 in decimal, DCLXV in Roman numerals, 1010011001 in binary, 0x299 in hexadecimal, and 0o1231 in octal. The Roman form DCLXV represents 500 + 100 + 50 + 10 + 5.

In the calendar, 665 is used as a year designation in both CE and BCE. 665 CE lies in the seventh century, while 665 BCE falls in the late first millennium BCE. Historical significance for those years depends on sources and context.

In science and technology, wavelengths near 665 nanometers lie in the red portion of the visible spectrum and are used in some red light-emitting diodes and signaling applications.