Number
28
- Positive
- Even
- Composite
- Perfect number
- 2 digits
28 is an even 2-digit integer and a composite number. It has 6 divisors and a digital root of 1.
Number properties
ParityEvendivisible by 2
SignPositive
PrimalityComposite
Absolute value28
Digit count2
Digit sum10
Digital root1repeated digit sum
PalindromicNoreads differently backwards
Triangular numberYes, the 7th triangular number
Happy numberYessquared-digit sums reach 1
Harshad numberNodivisible by its own digit sum
Factors & divisors
Prime factorisation2^2 × 7
Distinct prime factors22, 7
Number of divisors6
Sum of divisors σ(n)56
Aliquot sum28sum of the proper divisors
ClassificationPerfectthe aliquot sum equals the number
Euler's totient φ(n)12integers below n that share no factor with it
SquarefreeNohas a repeated prime factor
All divisors1, 2, 4, 7, 14, 286 in total
Arithmetic
Representations
Decimal28
Binary111005 bits
Octal34
Hexadecimal1C
Base 36S
Roman numeralXXVIII
In wordstwenty-eight
Ordinaltwenty-eighth
Scientific notation2.8 × 10^1
Engineering notation28
Nearest landmarks
Powers & multiples
28 to the power 2784
28 to the power 321,952
28 to the power 4614,656
28 to the power 517,210,368
First ten multiples28, 56, 84, 112, 140, 168, 196, 224, 252, 280
Powers of twoBetween 2^4 (16) and 2^5 (32)
Divisibility tests
Divisible by 2Yes
Divisible by 3No, remainder 1
Divisible by 4Yes
Divisible by 5No, remainder 3
Divisible by 6No, remainder 4
Divisible by 7Yes
Divisible by 8No, remainder 4
Divisible by 9No, remainder 1
Divisible by 10No, remainder 8
Divisible by 11No, remainder 6
Divisible by 12No, remainder 4
Divisible by 100No, remainder 28
Collatz (3n + 1) trajectory
Steps to reach 118the total stopping time
Reaches 1Yesthe Collatz conjecture says every positive integer does, but it is unproven
First steps28 → 14 → 7 → 22 → 11 → 34 → 17 → 52 → 26 → 13 → …
Last steps… → 5 → 16 → 8 → 4 → 2 → 1
As a percentage & fraction
As a percentage2,800%
28% as a decimal0.28
28% of 10028
28% of 1,000280
As a fraction of 10028/100
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