21189

Appears in sequences

• a(n) is the concatenation of n and 9n.at n=20A009474
• Interprimes (A024675) which are of the form s*prime, s=21.at n=38A075296
• Smallest multiple of n beginning with n and having a digit sum n, or 0 if no such number exists.at n=20A077727
• Let f(i) = prime( f(i - 1) (modulo 10^n) ) with f(0) = 1; a(n) is the term where the period of the sequence f(i) begins to repeat.at n=8A130172
• Lower Beatty array of sqrt(2).at n=35A182639
• Number of compositions (ordered partitions) of n into square pyramidal numbers (A000330).at n=37A322340
• Indices of the triangular numbers in A189475.at n=26A358417