A084021 - OEIS

%I #19 Nov 02 2025 10:18:00

%S 0,8,14,18,20,20,18,14,8,0,890,968,1044,1118,1190,1260,1328,1394,1458,

%T 1520,1580,1638,1694,1748,1800,1850,1898,1944,1988,2030,2070,2108,

%U 2144,2178,2210,2240,2268,2294,2318,2340,2360,2378,2394,2408,2420,2430,2438

%N Product of n and its 9's complement.

%C There are infinitely many nonzero squares in this sequence; for example a(10^(4n) - 10^(2n)) = (10^n*(10^(2n)-1))^2. - _David Wasserman_, Dec 07 2004

%H Seiichi Manyama, <a href="/A084021/b084021.txt">Table of n, a(n) for n = 0..9999</a>

%F a(n) = n*(10^d - 1 - n), where d is the number of digits in n.

%F a(n) = n*A061601(n). - _David Wasserman_, Dec 07 2004

%t Table[n(10^IntegerLength[n]-1-n),{n,0,50}] (* _Harvey P. Dale_, Nov 02 2025 *)

%o (PARI) {a(n) = n*(10^length(Str(n))-1-n)} \\ _Seiichi Manyama_, Sep 15 2019

%Y Cf. A061601.

%K base,easy,nonn

%O 0,2

%A _Amarnath Murthy_, May 23 2003

%E More terms from _David Wasserman_, Dec 07 2004

%E a(0)=0 prepended by _Seiichi Manyama_, Sep 15 2019