212121

Appears in sequences

• Decimal expansion of a(n) is given by the first n terms of the periodic sequence with initial period 2,1.at n=5A037495
• Triplets: base 10 representation is the juxtaposition of three identical strings.at n=20A074842
• Smallest multiple of n^2 beginning and ending in n, or 0 if no such multiple exists.at n=20A078211
• Integers are written in the form abcd...n where "a" means "At position a in this integer there is a digit b"; "b" means: "at position b there is a digit c"; "c" means: "at position c there is a digit d"; ... and "n" means: "at position n there is a digit a".at n=24A105956
• a(n) = (n in base 10) * (n in base 2).at n=21A127906
• Concatenate the terms of three copies of the Fibonacci sequence.at n=8A134554
• a(n) = 7*(10^(2*n) - 1)/33.at n=2A153848
• a(n) is the smallest number not already present that permits the cyclic repetition of the path 1,2 of the digits in the sequence.at n=12A165300
• List of imprimitive words over the alphabet {1,2}.at n=16A213972
• List of words over {1,2} with equal numbers of 1's and 2's.at n=22A214218
• Shortest (x+1,2x)-code of n.at n=14A233135
• Non-primitive words on {1,2,3}.at n=30A239018
• Pseudoprimes to base 6, written in base 6.at n=36A262103
• Underline all even terms; concatenations of underlined terms and of non-underlined terms both equal the concatenation of the entire sequence. This is the lexicographically earliest such sequence without duplicate terms and with an even digit among the first two terms.at n=28A307312
• Unique representation of nonnegative numbers by iterated tribonacci A, B and C sequences.at n=52A316713
• Integers k such that k^2 can be written as the sum of three positive fourth powers.at n=27A365657