A048544 Smallest oblong (promic) number containing exactly n 7's.

72, 11772, 771762, 757770, 77677782, 779777700, 7077772770, 778777127772, 7777770777090, 57777777767730, 7777577077877756, 177677777977777980, 17673737777477771772, 677777977779775677756, 1789771777777771777710, 7747977777770507777772, 298777877777767737777770

Offset: 1

Author

Patrick De Geest, May 15 1999

Keywords

• nonn
• base

Links

• Eric Weisstein's World of Mathematics, Pronic Number

Crossrefs

Cf. A048543, A002378.

Cf. A048530, A048532, A048534, A048536, A048538, A048540, A048542, A048546, A048548.

Programs

Mathematica

With[{pn=2*Accumulate[Range[430000000]]},Table[Select[pn,DigitCount[#,10,7] == n&,1],{n,12}]]//Flatten (* Harvey P. Dale, Aug 14 2017 *)

Python

def A048544(n):
    k, m = 1, 2
    while True:
        if str(m).count('7') == n:
            return m
        k += 1
        m += 2*k # Chai Wah Wu, Mar 23 2018

Extensions

a(13)-a(17) from Giovanni Resta, Oct 09 2019