555

Do you mean:

• the 555 timer IC (electronics), or
• the number 555 in math?

If you want the 555 timer quick reference:

• Pinout (8-pin DIP): 1 GND, 2 TRIG, 3 OUT, 4 RESET, 5 CTRL, 6 THR, 7 DISCH, 8 VCC
• Modes: astable (continuous pulses) and monostable (single pulse)
• Key formulas:
  • Astable frequency: f = 1.44 / ((R1 + 2R2) C)
  • Duty cycle: D = (R1 + R2) / (R1 + 2R2)
  • Monostable pulse width: t = 1.1 R C
• Quick example: R1 = 1 kΩ, R2 = 10 kΩ, C = 100 nF
  • f ≈ 1.44 / ((1k + 2×10k) × 100e-9) ≈ 686 Hz
  • D ≈ (1k + 10k) / (1k + 2×10k) ≈ 0.523 → ~52%

If you meant math:

• 555 = 3 × 5 × 37
• Roman numeral: 555 = DLV (500 + 50 + 5)
• Any other math aspects you want (properties, problems, prime factorization, etc.)?

Which would you like to dive into?