A048543 - OEIS

%I #29 Jul 25 2024 10:07:34

%S 8,108,878,870,8813,27924,84129,882483,2788865,7601169,88190572,

%T 421518419,4204014483,26034169427,42305694389,88022598108,

%U 546605779129,2788866736108,69048807207508,98844816642745,88187696578,2516,5270458024477168,26772616052167516,166654612230648203,521323093463907254

%N a(n) is the smallest value of m such that A002378(m), the m-th oblong number, contains exactly n 7's.

%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/PronicNumber.html">Pronic Number</a>

%e From Jesse Sealand, Oct 05 2019: (Start)

%e a(1) = 8 since A002378(8) = 72;

%e a(2) = 108 since A002378(36) = 11772;

%e a(3) = 878 since A002378(182) = 771762;

%e a(4) = 870 since A002378(1817) = 757770, etc.

%e (End).

%o (Python)

%o def A048543(n):

%o     k, m = 1, 2

%o     while True:

%o         if str(m).count('7') == n:

%o             return k

%o         k += 1

%o         m += 2*k # _Chai Wah Wu_, Mar 23 2018

%Y Cf. A048544, A002378.

%Y Cf. A048529, A048531, A048533, A048535, A048537, A048539, A048541, A048545, A048547.

%K nonn,base

%O 1,1

%A _Patrick De Geest_, May 15 1999

%E a(13)-a(16) from _Lars Blomberg_, Jun 10 2011

%E Name edited by _Jesse Sealand_, Oct 05 2019

%E a(17) from _Giovanni Resta_, Oct 09 2019

%E a(18)-a(25) from _Max Alekseyev_, Jul 25 2024