The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A308391 Number of ordered pairs of n-digit positive integers the product of which is a 2n-digit integer. 0
 58, 6610, 668843, 66965113, 6697324753, 669740590290, 66974140069358, 6697414817000983, 669741489800555031, 66974149061059480123 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS FORMULA a(n) = 9*10^(2n-1) - 10^n - Sum_{k=10^(n-1)+1..10^n-1} ceiling(10^(2n-1)/k). a(n) ~ (9-log(10))*10^(2n-1). EXAMPLE 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). PROG (Python) import math ende = 1 for i in range(1, 10):    anz = 0    for a in range(ende, 10*ende):       z = math.ceil((ende*ende*10)/a)       if z < ende*10:          anz = anz + ende*10 - z    ende = ende*10    print(i, anz) (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 CROSSREFS Cf. A174425. Sequence in context: A042625 A250919 A282438 * A128934 A173955 A243466 Adjacent sequences:  A308388 A308389 A308390 * A308392 A308393 A308394 KEYWORD nonn,base,more AUTHOR Reiner Moewald, May 23 2019 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified February 26 11:49 EST 2020. Contains 332279 sequences. (Running on oeis4.)