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Divisibility

  • An integer 𝑎 divides 𝑏 if there exists an integer 𝑐 such that 𝑏=𝑎𝑐
  • 𝑎∣𝑏 → “a divides b”
    • 2∣4 → 4 is divisible by 2
    • 8∤23 → 23 is not divisible by 8

Properties of Division

  • Distributive Property → if 𝑎∣𝑏 and 𝑎∣𝑐, then 𝑎∣(𝑏+𝑐)
  • Common Factor → if 𝑎∣𝑏, then 𝑎∣𝑏𝑐 for all integers 𝑐
  • Associative Property → if 𝑎∣𝑏 and 𝑏∣𝑐, then 𝑎∣𝑐

Remainder Theorem

  • 𝑚=𝑞𝑛+𝑟, 0≤𝑟≤𝑛
  • Where 𝑚 is the dividend, 𝑛 is the divisor, 𝑞 is the quotient, and 𝑟 is the remainder
  • 127÷4=31remainder3 → 127=31⋅4+3

Floor

  • Round down to the nearest integer
  • ⌊𝑥⌋=max{𝑛∈𝑍∣𝑛≤𝑥}
  • ⌊2.3⌋=2

Integer Division + Modulus

  • Integer Division → ⌊𝑎/𝑏⌋ only returns the quotient
    • Also known as “floor division”
  • Modulus → 𝑎mod𝑏 returns only the remainder

Congruence + Modular Arithmetic

  • If 𝑎≡𝑏mod𝑚, then 𝑎mod𝑚=𝑏mod𝑚
    • 17≡5mod12 because 17mod12=5

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