198 Jean-Marie De Koninck
18 620
the ninth number which is not perfect or multi-perfect but whose harmonic
mean is an integer (see the number 140).
18 737
the smallest number which can be written as the sum of two squares, as the
sum of two cubes and as the sum of two fourth powers and where each of the
numbers in question is distinct from the others: here 18 737 =
892
+
1042
=
173
+
243
=
84
+
114.
18 787
the second component of the largest known pair of prime numbers (p, q) such
that
pq−1
1 (mod
q2)
and
qp−1
1 (mod
p2);
here (p, q) = (2903, 18787) (see the number 2 903).
18 885
the
21rst
number n such that n ·
2n
1 is prime (see the number 115).
19 151
the sixth self contained number (see the number 293).
19 279
the number of Carmichael numbers 1013 (see the number 646).
19 333
the smallest prime number p such that p + 40 is prime and such that each
number between p and p + 40 is composite (see the number 139).
19 469
the 37th Lucas prime number (see the number 613).
19 521
the third 2-hyperperfect number (see the number 21).
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