Those Fascinating Numbers 397
was obtained by Selfridge in 1953, while the factor
6 487 031 809 = 395 937 ·
214
+ 1,
was obtained by Brillhart in 1962 (see the number 70 525 124 609).
2520(2521
1)
the 13th even perfect number, a 314 digit number.
11 . . . 1
317
the fourth prime number all of whose digits are 1 (see the number 19).
2606(2607
1)
the
14th
even perfect number, a 366 digit number.
21 279 1
the
15th
Mersenne prime, a 386 digit number.
2211
+ 1
the 12th Fermat number, a 617 digit number; it is a composite number, for
which two of its prime factors, namely
319 489 = 39 ·
213
+ 1 and 974 849 = 119 ·
213
+ 1,
were obtained by Cunningham in 1899 (see the number 70 525 124 609).
22 203
1
the 16th Mersenne prime, a 664 digit number.
22 281
1
the
17th
Mersenne prime, a 687 digit number.
21 278(21 279
1)
the 15th even perfect number, a 770 digit number.
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