21199

Appears in sequences

• Consider the mapping f(x/y) = (x+y)/(2xy) where x/y is a reduced fraction. Beginning with x_0 = 1 and y_0 = 2, repeated application of this mapping produces a sequence of fractions x_n/y_n; a(n) is the n-th numerator.at n=5A081475
• Numbers n such that phi(n) + pi(n) = n+1.at n=11A233189
• a(n) is the base-(-10) representation of a(n-1) read in base 10, starting from -1.at n=12A256639