It is a triangular number
In Roman numerals = XXI
It is the sun of the first 6 numbers (1+2+3+4+5+6=21)
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It is the 6th prime number.
26 = 64
composite even deficient
The number 11 is a prime number and it`s factors are 1 and 11
2 + 2 = 4. This is a fact.It is the first prime number.