555

Do you mean 555 in one of these contexts: math/number theory, or the 555 timer IC in electronics, or something else?

Quick notes you can use right away:

• In math: 555 = 3 × 5 × 37. So it’s composite (divisible by 3 and by 5). Also 555 = 15 × 37 and 555 is a Harshad number since 555 ÷ (5+5+5) = 37. In Roman numerals, 555 = DLV.
• In electronics: The 555 timer IC is a versatile timer/oscillator used in astable (oscillator) and monostable (one-shot) configurations, with applications in timers, PWM, and pulse generation.

If you tell me which context you’re studying and what you need (explanation, practice problems, or quick facts), I’ll tailor it.