A048543 a(n) is the smallest value of m such that A002378(m), the m-th oblong number, contains exactly n 7's.

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

Author

Patrick De Geest, May 15 1999

Keywords

• nonn
• base
• changed

Examples

			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)
		

Links

• Eric Weisstein's World of Mathematics, Pronic Number.

Crossrefs

Cf. A048544, A002378.

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

Programs

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

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