21321

Appears in sequences

• "DHK" (bracelet, identity, unlabeled) transform of 2,2,2,2,...at n=12A032251
• Decimal expansion of a(n) is given by the first n terms of the periodic sequence with initial period 2,1,3.at n=4A037638
• Denominators of continued fraction convergents to sqrt(549).at n=10A042051
• a(n) = 49*(n*(n+1)/2) + 6.at n=29A061792
• Partial sums of sequence (essentially A002378): 1, 2, 6, 12, 20, 30, 42, 56, 72, 90, ...at n=39A064999
• Triangular numbers which are a concatenation of two or more positive triangular numbers.at n=30A068144
• a(1) = 1; a(n) is the smallest triangular number > a(n-1) which differs from it at every digit.at n=29A068855
• Triangular numbers which are also happy numbers (cf. A007770).at n=32A076712
• Triangle read by rows in which the n-th row contains the n smallest triangular numbers with the least significant digits of the n-th triangular number.at n=19A095225
• Triangular numbers for which the sum of the digits is a square.at n=20A117404
• Triangular numbers composed of digits {1,2,3}.at n=6A119097
• Numbers k such that 6*p(k)*p(k+1)*p(k+2)*p(k+3)*p(k+4)-1 and 6*p(k)*p(k+1)*p(k+2)*p(k+3)*p(k+4)+1 are twin primes with p(h) = h-th prime.at n=40A129310
• Triangular numbers t which are average of two consecutive primes p and p+4.at n=23A129752
• Triangular numbers n*(n+1)/2 with n composite, where number of prime factors of n, counted with multiplicity, is less than the number of prime factors in n+1.at n=38A144524
• Numbers divisible by the sum of 5th powers of their digits.at n=39A169666
• Triangular numbers T such that T+2 is a prime.at n=38A171570
• Numbers whose absolute difference from a triangular number is never a prime.at n=38A292990
• Partial sums of A299266.at n=30A299267
• Number of nX5 0..1 arrays with every element equal to 0, 1, 3 or 5 horizontally, diagonally or antidiagonally adjacent elements, with upper left element zero.at n=8A302424
• Unique representation of nonnegative numbers by iterated tribonacci A, B and C sequences.at n=65A316713