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A308391 Number of ordered pairs of n-digit positive integers the product of which is a 2n-digit integer. 0

%I

%S 58,6610,668843,66965113,6697324753,669740590290,66974140069358,

%T 6697414817000983,669741489800555031,66974149061059480123

%N Number of ordered pairs of n-digit positive integers the product of which is a 2n-digit integer.

%F a(n) = 9*10^(2n-1) - 10^n - Sum_{k=10^(n-1)+1..10^n-1} ceiling(10^(2n-1)/k).

%F a(n) ~ (9-log(10))*10^(2n-1).

%e a(1)=58 since we get the following pairs: (2, 5), ..., (2, 9), (3, 4), ..., (3, 9), (4, 3), ..., (4, 9), (5, 2), ..., (5, 9), (6, 2), ..., (6, 9), (7, 2), ..., (7, 9), (8, 2), ..., (8, 9), (9, 2), ..., (9, 9).

%o (Python)

%o import math

%o ende = 1

%o for i in range(1,10):

%o anz = 0

%o for a in range(ende, 10*ende):

%o z = math.ceil((ende*ende*10)/a)

%o if z < ende*10:

%o anz = anz + ende*10 - z

%o ende = ende*10

%o print(i, anz)

%o (PARI) a(n) = 9*10^(2*n-1) - 10^n - sum(k=10^(n-1)+1, 10^n-1, ceil(10^(2*n-1)/k)); \\ _Michel Marcus_, Jun 25 2019

%Y Cf. A174425.

%K nonn,base,more

%O 1,1

%A _Reiner Moewald_, May 23 2019

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Last modified November 30 06:28 EST 2021. Contains 349419 sequences. (Running on oeis4.)