A285079 Oblong numbers the product of whose digits are positive oblong numbers.

2, 6, 12, 56, 132, 156, 756, 2756, 4556, 6162, 6972, 7656, 13572, 21756, 31152, 33672, 45156, 61752, 84972, 153272, 166872, 279312, 467172, 626472, 661782, 1273512, 1412532, 1541322, 1568756, 1596432, 1786232, 1867322, 2678132, 2817362, 3416952, 3521252

Offset: 1

Author

Melvin Peralta, Apr 09 2017

Keywords

• nonn
• base

Comments

Oblong numbers are numbers of the form k*(k+1) (A002378).

Links

• Robert Israel and Giovanni Resta, Table of n, a(n) for n = 1..391 (terms < 10^25, first 221 terms from Robert Israel)

Crossrefs

Cf. A002378, A053059, A069077.

Programs

Maple

filter:= proc(x) local t; t:= convert(convert(x,base,10),`*`);
t > 0 and issqr(1+4*t); end proc:
select(filter, [seq(x*(1+x),x=1..10^4)]); # Robert Israel, Apr 14 2017

Mathematica

f[x_] := Sqrt[1 + 4 (Times @@ IntegerDigits[x])]; Select[Table[n (n + 1), {n, 1, 10000}], f[#] > 1 && Mod[f[#], 2] == 1 &]