OFFSET
1,1
LINKS
Robert Israel, Table of n, a(n) for n = 1..10000
MAPLE
f:= proc(d) uses combinat; local S, Res, A1, A1p, A2, x1, x2;
Res:= NULL:
for S in choose({$0..6}, 3) do
x1:= S[3]*(7^d-1)/6;
for A1 in powerset(d) minus {{}} do
if S[1]=0 and member(d, A1) then next fi;
x2:= x1 + (S[1]-S[3])*add(7^(j-1), j=A1);
A1p:= {$1..d} minus A1;
for A2 in powerset(A1p) minus {{}, A1p} do
Res:= Res, x2 + (S[2]-S[3])*add(7^(j-1), j=A2);
od
od
od;
sort([Res]);
end proc:
op(f(3)), op(f(4)); # Robert Israel, Oct 29 2018
MATHEMATICA
Select[Range[150], Length[Union[IntegerDigits[#, 7]]]==3&] (* Harvey P. Dale, Mar 29 2019 *)
CROSSREFS
KEYWORD
nonn,base
AUTHOR
STATUS
approved