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28 (number)

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← 27 28 29 →
Cardinaltwenty-eight
Ordinal28th
(twenty-eighth)
Factorization22 × 7
Divisors1, 2, 4, 7, 14, 28
Greek numeralΚΗ´
Roman numeralXXVIII, xxviii
Binary111002
Ternary10013
Senary446
Octal348
Duodecimal2412
Hexadecimal1C16

28 (twenty-eight) is the natural number following 27 and preceding 29.

In mathematics

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The number 28 depicted as 28 balls arranged in a triangular pattern with the number of layers of 7
28 as the sum of four nonzero squares.

28 is a composite number, a perfect number,[1] a harmonic divisor number,[2] a centered nonagonal number,[3] a hexagonal number,[4] a happy number,[5] and a triangular number.[6]

28 also appears in the Padovan sequence.[7]

In algebraic geometry, 28 is the number of bitangents to a general plane quartic curve.

In science

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28 is the fourth magic number in physics.

References

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  1. ^ "Sloane's A000396 : Perfect numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved May 31, 2016.
  2. ^ "Sloane's A001599 : Harmonic numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved July 17, 2026.
  3. ^ "Sloane's A060544 : Centered 9-gonal numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved July 17, 2026.
  4. ^ "Sloane's A000384 : Hexagonal numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved July 17, 2026.
  5. ^ "Sloane's A007770 : Happy numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved May 31, 2016.
  6. ^ "Sloane's A000217 : Triangular numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved July 16, 2026.
  7. ^ "Sloane's A000931 : Padovan sequence". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved May 31, 2016.
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