Numbers /21
How rare is 21?
21 scores 1,283 and sits in the Singular tier — 1 in 100,000 of all supported numbers score this high or higher. Everything below is computed from a complete analysis of all 1,000,001 numbers in range.
RNGdle.net’s own rarity score. Top <0.01% means that proportion of supported numbers scores at least this high. Every exact number has the same chance of being rolled. How this differs from EP · Badge guide
These notes make the number easy to read, but they add no score unless a separately counted rarity signal appears below.
Appears in the Fibonacci sequence.
Factorization · signal frequency
This number's digits add up to an uncommon total.
Arithmetic · signal frequency
One of only 90 two-digit numbers in range.
Range · signal frequency
Its binary form reads the same both ways.
Representation · signal frequency
This number uses an uncommon count of distinct digits.
Repetition · signal frequency
A sum 1 + 2 + … + k for some k.
Factorization · signal frequency
Divisible by the sum of its digits.
Arithmetic · signal frequency
The product of exactly two primes.
Factorization · signal frequency
Strongest signal: Fibonacci number. It appears in 30 supported numbers and contributes +300 after correlation decay.
Why is 21 unusual? Only <0.01% of the 1,000,001 supported numbers receive an equal or higher rarity score. This number falls in the Singular tier. Scoring is grounded in the exact frequency of each detected trait — rarer traits carry more information and contribute more to the score.
Algorithm version 1.1.0
Patterns in this number
Each of these has its own page explaining how common the pattern is and why.
Harshad Numbers
A Harshad — or Niven — number is divisible by the sum of its own digits. 18 works because 1 + 8 = 9 and 18 ÷ 9 = 2.
Semiprime Numbers
A semiprime is a product of exactly two primes, counted with multiplicity — 15 = 3 × 5, and 49 = 7 × 7 counts too.
Fibonacci Numbers
Each Fibonacci number is the sum of the two before it: 1, 1, 2, 3, 5, 8, 13 … reaching 832040 before the range runs out.
Triangular Numbers
A triangular number counts the dots in a triangle: 1, 3, 6, 10, 15, 21 — each one the running total 1 + 2 + … + k.
Binary Palindromes
Written in base two, these numbers read the same forwards and backwards. 9 is 1001, and 585 is 1001001001.