excel has no extrapolation function built in.
That is not true i just typed your values in a column, marked the whole column and extended it to 100 values, excell showed the extrapolated values instantly.
1
2
3
5
7
9
15
19
18.92857143
21.44047619
23.95238095
26.46428571
28.97619048
31.48809524
34
36.51190476
39.02380952
41.53571429
44.04761905
46.55952381
49.07142857
51.58333333
54.0952381
56.60714286
59.11904762
61.63095238
64.14285714
66.6547619
69.16666667
71.67857143
etc...

Regards
Mahmood