text: electronic signature or through electronic data interchange. 
  
  Reception of invoices by standard format (pdf) or email is not accepted. 
  
  **II.20. Price revision**
  
  If a price revision index is provided in Article I.5.2, this Article applies to it. 
  
  Prices are fixed and not subject to revision during the first year of the FWC. 
  
  At the beginning of the second and every following year of the FWC, each price may be 
  revised upwards or downwards at the request of one of the parties. 
  
  A party may request a price revision in writing no later than three months before the 
  anniversary date of entry into force of the FWC. The other party must acknowledge the 
  request within 14 days of receipt. 
  
  At the anniversary date, the contracting authority must communicate the final index for 
  the month in which the request was received, or failing that, the last provisional index 
  available for that month. The contractor establishes the new price on this basis and 
  communicates it as soon as possible to the contracting authority for verification. 
  
  The contracting authority purchases on the basis of the prices in force at the date on which 
  the specific contract enters into force. 
  
  The price revision is calculated using the following formula: 