1 1
2 3
3 7
4 12
5 17
6 24
7 32
8 41
9 51
10 61
11 73
12 85
13 98

a(11) >= 73
a(12) = 85 
a(13) = 98 
a(14) >= 112
a(15) >= 127
a(16) >= 142
a(17) >= 158
a(18) >= 174
a(19) >= 192
a(20) >= 210