21899

Appears in sequences

• Numbers k such that the period of the continued fraction for sqrt(k) contains exactly 19 ones.at n=18A031787
• Engel expansion of the twin primes constant ~ .660161815846869573927812110014555778432623360284733413319448.at n=9A096189
• a(1)=3; for n > 1, a(n) = 1 + a(n-1) + gcd( a(n-1)*(a(n-1)+2), A073829(a(n-1)) ).at n=31A167053
• Number of (n+1) X (n+1) -4..4 symmetric matrices with every 2 X 2 subblock having sum zero and one or two distinct values.at n=17A211490
• Number of free highly irregular trees with n nodes.at n=41A259864
• a(n) = A007678(2*n)/(2*n).at n=41A341734
• Fixed points in A375564.at n=26A376584
• Numerator of the sum of the reciprocals of all square divisors of all positive integers <= n.at n=33A384817