1821 is a number that yields a prime whenever 1 is inserted anywhere in it, including at the beginning or end (A068679 and A216165).
1821 is a centered icosagonal number (A069133). It is also a concentric decagonal number (A195142).
1821 is the number of primes less than the cube of 25 (A038098).
1821 is 2111110 in base 3. It is 130131 in base 4 and 3435 in base 8. 1821 is 24241 in base 5.
Source: OEIS
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