A120154 a(1)=9; a(n)=floor((27+sum(a(1) to a(n-1)))/3).

9, 12, 16, 21, 28, 37, 50, 66, 88, 118, 157, 209, 279, 372, 496, 661, 882, 1176, 1568, 2090, 2787, 3716, 4955, 6606, 8808, 11744, 15659, 20879, 27838, 37118, 49490, 65987, 87983, 117310, 156414, 208552, 278069, 370759, 494345, 659127