how to count and write no of isomers

This is the sequence for the number of isomers present for an alkane with n carbon atoms (n ranges from 1 to 20 below):

1, 1, 1, 2, 3, 5, 9, 18, 35, 75, 159, 355, 802, 1858, 4347, 10359, 24894, 60523, 148284, 366319, ...

CH4 has 1 isomer
C2H6 has 1 isomer
C3H8 has 1 isomer
C4H10 has 2 isomers
C5H12 has 3 isomers
C6H14 has 5 isomers
C7H16 has 9 isomers
C8H18 has 18 isomers
C9H20 has 35 isomers
...and so on

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It has not got a specific formula. You just have to remember

This is the sequence for the number of isomers present for an alkane with n carbon atoms (n ranges from 1 to 20 below):

1, 1, 1, 2, 3, 5, 9, 18, 35, 75, 159, 355, 802, 1858, 4347, 10359, 24894, 60523, 148284, 366319, ...

CH4 has 1 isomer
C2H6 has 1 isomer
C3H8 has 1 isomer
C4H10 has 2 isomers
C5H12 has 3 isomers
C6H14 has 5 isomers
C7H16 has 9 isomers
C8H18 has 18 isomers
C9H20 has 35 isomers