Compare 5 vs 21

Unique factors12
Total factors12
Prime factorization5131 × 71
Prime factorization53 × 7
Divisors count24
Divisors1, 51, 3, 7, 21
Number of properties361
Additive primes3rd 
Bell primes2nd 
Centered square primes1st 
Chen primes3rd 
Class 1+ primes3rd 
Dihedral primes2nd 
Factorial primes3rd 
Fermat primes2nd 
Fibonacci primes3rd 
Good primes1st 
Harmonic primes1st 
Left-truncatable primes3rd 
Multiplicative primes3rd 
Palindromic primes3rd 
Pierpont primes3rd 
Pierpont primes of the 2nd kind3rd 
Prime quadruplets (1st member)1st 
Prime quintuplet 2s (1st member)1st 
Prime triplets (1st member)1st 
Proth primes2nd 
Pythagorean primes1st 
Restricted left-truncatable primes2nd 
Right-truncatable primes3rd 
Safe primes1st 
Semiprimes 7th
Sexy primes (1st member)1st 
Sexy prime quadrupletss (1st member)1st 
Sexy prime triplets (1st member)1st 
Solinas primes2nd 
Sophie germain primes3rd 
Super primes2nd 
Thabit primes2nd 
Twin primes (1st member)2nd 
Twin primes (2nd member)1st 
Two-sided primes3rd 
Wilson primes1st 
Roman numberalsVXXI
Base 21012101012
Base 31232103
Base 41141114
Base 5105415
Base 656336
Base 757307
Base 858258
Base 959239
Base 105102110
Base 115111a11
Base 125121912
Base 135131813
Base 145141714
Base 155151615
Base 165161516
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