Smallest positive integer not yet appeared so that the sum of the first n elements is divisible by n
Python 2, 43 bytes
r=.5+5**.5/2
lambda n:[n/r+1,n*r][n%r&lt;1]//1

Outputs floats. Uses the relation

a(n) = A002251(n-1) + 1

to Wythoff pairs. Takes from the upper or lower Beatty sequence by multiplying or dividing by the golden ratio and converting to an integer.