282 Jean-Marie De Koninck

2 199 978

• the eighth number which is not a palindrome, but which divides the number

obtained by reversing its digits (see the number 1 089).

2 230 271

• the seventh solution of σ2(n) = σ2(n + 2) (see the number 1 079).

2 424 833

• the smallest prime factor of the Fermat number F9 =

229

+ 1, whose complete

factorization is given by

F9 = 2424833 · 7455602825647884208337395736200454918783366342657 · P99.

2 469 601 (=

192

· 6841)

• the third 18-hyperperfect number (see the number 1 333).

2 481 757

• the second prime number p such that

23p−1

≡ 1 (mod

p2);

see the number

13.

2 500 704

• the third and largest known dihedral 3-perfect number (see the number 5 472).

2 562 624

• the smallest number which can be written as the sum of three distinct cubes

in eight distinct ways:

2 562 624 =

83

+

363

+

1363

=

83

+

643

+

1323

=

123

+

1003

+

1163

=

173

+

463

+

1353

=

303

+

1033

+

1133

=

363

+

603

+

1323

=

513

+

853

+

1223

=

693

+

723

+

1233

(see the number 1 009).

2 568 226

• the third even number n for which

2n

≡ 2 (mod n); see the number 161 038.