555

Do you mean the number 555 (math) or the 555 timer IC (electronics), or something else?

If you want math quick-start with 555:

• Prime factorization: 555 = 3 × 5 × 37
• Divisors: 1, 3, 5, 15, 37, 111, 185, 555 (8 divisors)
• Sum of divisors: 912
• Digital root: 6 (since 5+5+5 = 15 → 1+5 = 6)
• Roman numeral: DLV

If you want electronics on the 555 timer (brief):

• Key idea: has comparators at 2/3 Vcc and 1/3 Vcc, plus a flip-flop and a discharge transistor.
• Common modes: astable (free-running) and monostable (one-shot). Formulas are simple but depend on R1, R2, C, etc.

Tell me which you’d like, and I’ll tailor some problems or a quick refresher.