210021

Appears in sequences

• Smallest multiple of the n-th prime beginning and ending in n with a(1)=a(3)=0.at n=20A078212
• Numbers encoded in an alternate, sometimes more compact, binary system with a third, dual-purpose, prefix/delimiter symbol not always required.at n=29A140116
• Consider a number of k digits n = d_(k)*10^(k-1) + d_(k-1)*10^(k-2) + … + d_(2)*10 + d_(1). Sequence lists the numbers n such that Sum_{i=1..k-1}{phi(Sum_{j=1..i}{d_(j)*10^(j-1)})} = Sum_{i=1..k-1}{sigma(Sum_{j=1..i}{d_(k-j+1)*10^(i-j)})} (see example below).at n=29A244069
• Expansion of sqrt((1-x) / (1-5*x)^5).at n=6A387228