Compare 21 vs 31
| Property | 21 | 31 |
|---|---|---|
| Type | semiprime | prime |
| Unique factors | 2 | 1 |
| Total factors | 2 | 1 |
| Prime factorization | 31 × 71 | 311 |
| Prime factorization | 3 × 7 | 31 |
| Divisors count | 4 | 2 |
| Divisors | 1, 3, 7, 21 | 1, 31 |
| Number of properties | 1 | 18 |
| Centered decagonal primes | 2nd | |
| Centered triangular primes | 2nd | |
| Chen primes | 11th | |
| Class 1+ primes | 8th | |
| Emirps | 3rd | |
| Euclid primes | 4th | |
| Happy primes | 5th | |
| Lucky primes | 4th | |
| Mersenne primes | 3rd | |
| Multiplicative primes | 7th | |
| Pierpont primes of the 2nd kind | 8th | |
| Primes | 11th | |
| Right-truncatable primes | 7th | |
| Semiprimes | 7th | |
| Sexy primes (1st member) | 7th | |
| Sexy prime triplets (1st member) | 5th | |
| Solinas primes | 10th | |
| Super primes | 5th | |
| Twin primes (2nd member) | 5th | |
| Roman numberals | XXI | XXXI |
| Base 2 | 101012 | 111112 |
| Base 3 | 2103 | 10113 |
| Base 4 | 1114 | 1334 |
| Base 5 | 415 | 1115 |
| Base 6 | 336 | 516 |
| Base 7 | 307 | 437 |
| Base 8 | 258 | 378 |
| Base 9 | 239 | 349 |
| Base 10 | 2110 | 3110 |
| Base 11 | 1a11 | 2911 |
| Base 12 | 1912 | 2712 |
| Base 13 | 1813 | 2513 |
| Base 14 | 1714 | 2314 |
| Base 15 | 1615 | 2115 |
| Base 16 | 1516 | 1f16 |