when did they stop using letters in phone numbers
Telephone number In the most areas of North America, telephone numbers in metropolitan communities consisted of a combination of digits and letters, starting in the 1920s until the 1960s. Letters were translated to dialed digits, a mapping that was displayed directly on the telephone dial. Each of the digits 2 to 9, and sometimes 0, corresponded to a group of typically three letters. The leading two or three letters of a telephone number indicated the exchange name, for example, EDgewood and IVanhoe, and were followed by 5 or 4 digits. The limitations that these system presented in terms of usable names that were easy to distinguish and spell, and the need for a comprehensive numbering plan that enabled direct-distance dialing, led to the introduction of all-number dialing in the 1960s.
Social Security number The last four digits are serial numbers. They represent a straight numerical sequence of digits from 0001 to 9999 within the group.
1 + 2 + 3 + 4 + ⋯ which increases without bound as n goes to infinity. Because the sequence of partial sums fails to converge to a finite limit, the series does not have a sum.
Marcus Álvarez