Number
15
- Positive
- Odd
- Composite
- 2 digits
15 is an odd 2-digit integer and a composite number. It has 4 divisors and a digital root of 6.
Number properties
ParityOddnot divisible by 2
SignPositive
PrimalityComposite
Absolute value15
Digit count2
Digit sum6
Digital root6repeated digit sum
PalindromicNoreads differently backwards
Triangular numberYes, the 5th triangular number
Happy numberNosquared-digit sums reach 1
Harshad numberNodivisible by its own digit sum
Factors & divisors
Prime factorisation3 × 5
Distinct prime factors23, 5
Number of divisors4
Sum of divisors σ(n)24
Aliquot sum9sum of the proper divisors
ClassificationDeficientthe aliquot sum is less than the number
Euler's totient φ(n)8integers below n that share no factor with it
SquarefreeYesno repeated prime factor
All divisors1, 3, 5, 154 in total
Arithmetic
Representations
Decimal15
Binary11114 bits
Octal17
HexadecimalF
Base 36F
Roman numeralXV
In wordsfifteen
Ordinalfifteenth
Scientific notation1.5 × 10^1
Engineering notation15
Nearest landmarks
Powers & multiples
15 to the power 2225
15 to the power 33,375
15 to the power 450,625
15 to the power 5759,375
First ten multiples15, 30, 45, 60, 75, 90, 105, 120, 135, 150
Powers of twoBetween 2^3 (8) and 2^4 (16)
Divisibility tests
Divisible by 2No, remainder 1
Divisible by 3Yes
Divisible by 4No, remainder 3
Divisible by 5Yes
Divisible by 6No, remainder 3
Divisible by 7No, remainder 1
Divisible by 8No, remainder 7
Divisible by 9No, remainder 6
Divisible by 10No, remainder 5
Divisible by 11No, remainder 4
Divisible by 12No, remainder 3
Divisible by 100No, remainder 15
Collatz (3n + 1) trajectory
Steps to reach 117the total stopping time
Reaches 1Yesthe Collatz conjecture says every positive integer does, but it is unproven
First steps15 → 46 → 23 → 70 → 35 → 106 → 53 → 160 → 80 → 40 → …
Last steps… → 5 → 16 → 8 → 4 → 2 → 1
As a percentage & fraction
As a percentage1,500%
15% as a decimal0.15
15% of 10015
15% of 1,000150
As a fraction of 10015/100
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