wrote:
I have an ultrabook and hence do not have a
Numlock or a Number pad option. Any other method ?

I have the impression, perhaps incorrectly, that Excel on Ultrabook is not
fully-functional. Is that right?

Try the following. Click on the Insert tab on the ribbon, then click on
Symbol on the right, select Mathematical Operators from the Subset drop-down
menu, select the (poor-looking) infinity symbol, then click on Insert. If
you are done, click on Close, then press Enter to complete the text input.