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Number bases

Converting between decimal, binary, octal and hexadecimal.

A base-b number is a sum of powers of b. To convert from decimal, divide by b repeatedly; the remainders, read from the last to the first, are the digits. Binary (base 2) and hexadecimal (base 16) are how computers store every number you see.

Toimiva esimerkki: 255 to binary

Number-Theory · Number-Bases

255 to binary

255,\ 2

Askel kerrallaan

  1. 255_{10} \to \text{base } 2

    Repeatedly divide by 2; the remainders, read bottom-up, are the digits.

  2. 255 = 127 \times 2 + 1

    remainder 1

  3. 127 = 63 \times 2 + 1

    remainder 1

  4. 63 = 31 \times 2 + 1

    remainder 1

  5. 31 = 15 \times 2 + 1

    remainder 1

  6. 15 = 7 \times 2 + 1

    remainder 1

  7. 7 = 3 \times 2 + 1

    remainder 1

  8. 3 = 1 \times 2 + 1

    remainder 1

  9. 1 = 0 \times 2 + 1

    remainder 1

Paljasta vastaus
255_{10} = 11111111_{2}

Symbols used here

\log_b x,\ \ln x
logarithm, natural log
The exponent b must be raised to for x; ln uses base e.
\sum_{k=1}^{n} a_k
summation
Add a_k for k = 1 up to n.
\prod_{k=1}^{n} a_k
product
Multiply a_k for k = 1 up to n.
\mathbb{N},\ \mathbb{Z},\ \mathbb{Q},\ \mathbb{R},\ \mathbb{C}
number sets
Naturals, integers, rationals, reals, complex numbers.
a \equiv b \pmod n
congruent modulo n
n divides a − b; a and b have the same remainder.
a \mid b,\ \gcd(a,b)
divides, greatest common divisor
b is a multiple of a; the largest number dividing both.
\varphi(n),\ \pi(x)
Euler's totient, prime-counting function
Count of 1..n coprime to n; number of primes up to x.
\mathbb{Z}/n\mathbb{Z},\ \mathbb{Z}_n
integers modulo n
The remainders 0…n−1 with clock arithmetic.
a \bmod n
remainder
What is left after dividing a by n.

How to: Number bases

  1. Repeatedly divide by 2; the remainders, read bottom-up, are the digits.
  2. remainder 1
  3. remainder 1
  4. remainder 1
  5. remainder 1
  6. remainder 1
  7. remainder 1
  8. remainder 1

Questions people ask

Why are primes so important?

Every integer factors into primes in exactly one way, so primes are the atoms of multiplication. Cryptography relies on that factoring being easy to state and hard to do.

How do I tell whether a big number is prime?

Trial division up to the square root works for small numbers. For large ones, probabilistic tests (Miller–Rabin) give an answer that is wrong with negligible probability, and deterministic tests (AKS) exist but are slower.

Kokeile omaasi

Lisää Number Theory

Prime factorisationPrime numbersGCD and LCMModular arithmeticDivisorsSequencesDiophantine equationsFermat's little theorem and Euler's theoremRSA: cryptography from number theory