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URL: https://oeis.org/A048543

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A048543
a(n) is the smallest value of m such that A002378(m), the m-th oblong number, contains exactly n 7's.
12
8, 108, 878, 870, 8813, 27924, 84129, 882483, 2788865, 7601169, 88190572, 421518419, 4204014483, 26034169427, 42305694389, 88022598108, 546605779129, 2788866736108, 69048807207508, 98844816642745, 88187696578, 2516, 5270458024477168, 26772616052167516, 166654612230648203, 521323093463907254
OFFSET
1,1
LINKS
Eric Weisstein's World of Mathematics, Pronic Number.
EXAMPLE
From Jesse Sealand, Oct 05 2019: (Start)
a(1) = 8 since A002378(8) = 72;
a(2) = 108 since A002378(36) = 11772;
a(3) = 878 since A002378(182) = 771762;
a(4) = 870 since A002378(1817) = 757770, etc. (End)
PROG
(Python)
def A048543(n):
k, m = 1, 2
while True:
if str(m).count('7') == n:
return k
k += 1
m += 2*k # Chai Wah Wu, Mar 23 2018
KEYWORD
nonn,base
AUTHOR
Patrick De Geest, May 15 1999
EXTENSIONS
a(13)-a(16) from Lars Blomberg, Jun 10 2011
Name edited by Jesse Sealand, Oct 05 2019
a(17) from Giovanni Resta, Oct 09 2019
a(18)-a(25) from Max Alekseyev, Jul 25 2024
STATUS
approved