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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.

%I A048543 #34 Sep 02 2025 04:09:31
%S A048543 8,108,878,870,8813,27924,84129,882483,2788865,7601169,88190572,
%T A048543 421518419,4204014483,26034169427,42305694389,88022598108,
%U A048543 546605779129,2788866736108,69048807207508,98844816642745,88187696578,2516,5270458024477168,26772616052167516,166654612230648203,521323093463907254
%N A048543 a(n) is the smallest value of m such that A002378(m), the m-th oblong number, contains exactly n 7's.
%H A048543 Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/PronicNumber.html">Pronic Number</a>.
%e A048543 From _Jesse Sealand_, Oct 05 2019: (Start)
%e A048543 a(1) = 8 since A002378(8) = 72;
%e A048543 a(2) = 108 since A002378(36) = 11772;
%e A048543 a(3) = 878 since A002378(182) = 771762;
%e A048543 a(4) = 870 since A002378(1817) = 757770, etc. (End)
%o A048543 (Python)
%o A048543 def A048543(n):
%o A048543     k, m = 1, 2
%o A048543     while True:
%o A048543         if str(m).count('7') == n:
%o A048543             return k
%o A048543         k += 1
%o A048543         m += 2*k # _Chai Wah Wu_, Mar 23 2018
%Y A048543 Cf. A048544, A002378.
%Y A048543 Cf. A048529, A048531, A048533, A048535, A048537, A048539, A048541, A048545, A048547.
%K A048543 nonn,base,changed
%O A048543 1,1
%A A048543 _Patrick De Geest_, May 15 1999
%E A048543 a(13)-a(16) from _Lars Blomberg_, Jun 10 2011
%E A048543 Name edited by _Jesse Sealand_, Oct 05 2019
%E A048543 a(17) from _Giovanni Resta_, Oct 09 2019
%E A048543 a(18)-a(25) from _Max Alekseyev_, Jul 25 2024