SOCIAL SECURITY FOR MIGRANT WORKERS - INVALIDITY INSURANCE - INSURANCE PERIODS COMPLETED - BENEFITS - CALCULATION - THEORETICAL AMOUNT - REDUCTION - NATIONAL RULE - APPLICATION WHERE PAYMENT IS MADE TO THE PERSON CONCERNED FROM A SOURCE OUTSIDE THE COMMUNITY ( REGULATION NO 3 OF THE COUNCIL , ARTICLE 11 ( 2 ), ARTICLE 26 , ARTICLE 28 ; REGULATION NO 4 OF THE COUNCIL , ARTICLE 9 ( 2 ))