Those Fascinating Numbers 341

1 684 126 080 (= 27 · 35 · 5 · 72 · 13 · 17)

• the third solution of

σ(n)

n

=

19

4

(see the number 746 444 160).

1 716 728 832 (=

210

·

32

· 7 · 11 · 13 · 23 · 89)

• the fifth solution of

σ(n)

n

=

15

4

(see the number 293 760).

1 767 263 190

• the

19th

Catalan number (see the number 14).

1 870 585 220

• the number of twin prime pairs

1012

(see the number 1 224).

1 879 906 755

• the fifth number which is not the square of a prime number, but which can be

written as the sum of the squares of some of its prime factors: here 1 879 906 755 =

3 · 5 · 2897 · 43261 = 52 + 28972 + 432612 (see the number 870).

1 903 757 312

• the 15th Euler number (see the number 272).

1 934 917 632

• the smallest fifth power which can be written as the sum of five fifth powers:

1 934 917 632 =

725

=

195

+

435

+

465

+

475

+

675

(Lander and Parkin, 1967).

1 977 225 901 (=

75

· 117 643)

• the fourth 6-hyperperfect number (see the number 301).

2 012 783 315

• the smallest number n which allows the sum

i≤n

1

i

to exceed 22 (see the number

83).