238 Jean-Marie De Koninck

114 689

• the smallest prime factor of F12 =

2212

+ 1, which in particular is the smallest

Fermat number for which the complete factorization is not yet known (see the

number 70 525 124 609).

115 921 (= 13 · 37 · 241)

• the fourth pseudoprime in bases 2, 3, 5 and 7 (see the number 29 341).

115 975

• the tenth Bell number (see the number 52).

119 972

• the smallest Niven number n such that n +40 is also a Niven number, but with

no others in between (see the number 28 680).

120 120 (=

23

· 3 · 5 · 7 · 11 · 13)

• the smallest number n such that Ω(n)ω(n) 2n: here Ω(n)ω(n)/n ≈ 2.18235;

the sequence of numbers satisfying this inequality begins as follows: 120 120,

240 240, 480 480, 1 021 020, 2 042 040, . . . (to discover the smallest number n

such that

Ω(n)ω(n)

n, see the number 60; as for the maximal value of the

quantity

Ω(n)ω(n)/n,

see the number 3 569 485 920);

• the smallest number n such that σ(n) = 21 · φ(n) (see the number 416 640).

121 056

• the fifth number n such that φ(n)σ(n) is a fourth power: φ(121 056)σ(121 056) =

3364 (see the number 170).

123 653

• the largest known prime number p such that

12p−1

≡ 1 (mod

p2):

the only

other prime number p

232

satisfying this property is 2 693 (see Ribenboim

[169], p. 347).

128 205

• the second number which quadruples when its last digit is moved in first position

(see the number 102 564).