formula on a mathematical problom

[yamapinews]

Column A are number arranged in sequence, I need a formula than can be rolled down to show a sub-sequence like below
[TABLE="width: 146"]
<colgroup><col width="74" style="width:56pt"> <col width="72" style="width:54pt"> </colgroup><tbody>[TR]
[TD="class: xl522, width: 74"]ColA[/TD]
[TD="class: xl523, width: 72"]ColB[/TD]
[/TR]
[TR]
[TD="class: xl524, width: 74, align: right"]1[/TD]
[TD="class: xl523, align: right"]1.1[/TD]
[/TR]
[TR]
[TD="class: xl524, width: 74, align: right"]1[/TD]
[TD="class: xl523, align: right"]1.2[/TD]
[/TR]
[TR]
[TD="class: xl524, width: 74, align: right"]2[/TD]
[TD="class: xl523, align: right"]2.1[/TD]
[/TR]
[TR]
[TD="class: xl524, width: 74, align: right"]3[/TD]
[TD="class: xl523, align: right"]3.1[/TD]
[/TR]
[TR]
[TD="class: xl524, width: 74, align: right"]3[/TD]
[TD="class: xl523, align: right"]3.2[/TD]
[/TR]
[TR]
[TD="class: xl524, width: 74, align: right"]3[/TD]
[TD="class: xl523, align: right"]3.3[/TD]
[/TR]
[TR]
[TD="class: xl524, width: 74, align: right"]4[/TD]
[TD="class: xl523, align: right"]4.1[/TD]
[/TR]
[TR]
[TD="class: xl524, width: 74, align: right"]5[/TD]
[TD="class: xl523, align: right"]5.1[/TD]
[/TR]
[TR]
[TD="class: xl524, width: 74, align: right"]6[/TD]
[TD="class: xl523, align: right"]6.1[/TD]
[/TR]
[TR]
[TD="class: xl524, width: 74, align: right"]7[/TD]
[TD="class: xl523, align: right"]7.1[/TD]
[/TR]
[TR]
[TD="class: xl524, width: 74, align: right"]7[/TD]
[TD="class: xl523, align: right"]7.2[/TD]
[/TR]
</tbody>[/TABLE]

 

[footoo]

With the numbers starting in A2 :
=IF(A2=A1,B1+0.1,A2+0.1)

 

[Tetra201]

Maybe

=--(A1&"."&COUNTIF($A$1:A1,A1))