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5 (number)

 

5 (number)


5 (five) is the natural number following 4 and preceding 6.

The SI prefix for 10005 is peta (P), and for its reciprocal femto (f).

Evolution of the glyph

The evolution of our modern glyph for five cannot be neatly traced back to the Brahmin Indians quite the same way it can for 1 to 4. Later on the Kushana and Gupta Indians had among themselves several different glyphs which bear no resemblance to the modern glyph. The Nagari and Punjabi took these glyphs and all came up with glyphs that look like a lowercase "h" mirrored and turned upside down. The Ghubar Arabs transformed the glyph in several different ways, coming up with glyphs that look more like 4s or 3s than 5s. It was from those characters that the Europeans finally came up with the modern 5, though from purely graphical evidence, it would be much easier to conclude that our modern 5 came from the Khmer.

In mathematics


Five is the third smallest prime number, after 2 and 3, and before 7. Because it can be written as 2^(2^1)+1, five is classified as a Fermat prime. 5 is the third Mersenne prime exponent. Five is the second Wilson prime.

Five is the only prime number to end in the digit 5, because all other numbers written with a 5 in the ones-place under the decimal system are multiples of five. As a consequence of this, 5 is in base 10 a 1-automorphic number.

Five is a factor of 10, so fractions with 5 in the denominator do not yield infinite decimal expansions, unlike most other primes. When written in the decimal system, all multiples of 5 will end in either 5 or 0.

The number 5 is a Fibonacci number, being 2 plus 3. The next Fibonacci number is 8. 5 is also a Pell number.

While polynomial equations of degree 4 and below can be solved with radicals, equations of degree 5 and higher cannot generally be so solved. This is the Abel-Ruffini theorem. This is related to the fact that the symmetric group Sn is a solvable group for n ≤ 4 and not solvable for n ≥ 5.

While all graphs with 4 or fewer vertices are planar, there exists a graph with 5 vertices which is not planar: K5, the complete graph with 5 vertices.

Five is also the number of Platonic solids.

A polygon with five sides is a pentagon. Figurate numbers representing pentagons (including five) are called pentagonal numbers. Five is also a square pyramidal number.

Five is the answer to the question asked at the very end of the mathematics quiz show in the movie Little Man Tate. (Our young protagonist blurts out the answer, but the host mishears it as being the answer from the contestant to whom the question is posed, and declares him the winner.)

In numbering systems

  • In binary code five is 101
  • In ternary code five is 12
  • In quaternary numeral system code five is 11
  • In quinary five is 10; in senary code and all codes above (such as octal, decimal and hexadecimal) five is 5.
  • The Roman numeral for five is V, which comes from a representation of an outstretched hand.
  • In the Greek alphabet, ε (epsilon) has numerical value of 5.
  • In the Hebrew alphabet, ה (heh) has numerical value of 5.
  • In the Cyrillic alphabet, Е has numerical value of 5.
  • In the Glagolitic alphabet, (dobro) has numerical value of 5.
  • The kanji and Chinese character for five are both 五, and its formal writing in Chinese is 伍 (pinyin wǔ).

    In science



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