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A257849 a(n) = floor(n/9) * (n mod 9).

%I A257849 #33 Jan 19 2023 10:25:28
%S A257849 0,0,0,0,0,0,0,0,0,0,1,2,3,4,5,6,7,8,0,2,4,6,8,10,12,14,16,0,3,6,9,12,
%T A257849 15,18,21,24,0,4,8,12,16,20,24,28,32,0,5,10,15,20,25,30,35,40,0,6,12,
%U A257849 18,24,30,36,42,48,0,7,14,21,28,35,42,49,56,0,8,16,24,32,40,48,56,64,0
%N A257849 a(n) = floor(n/9) * (n mod 9).
%C A257849 Equivalently, write n in base 9, multiply the last digit by the number with its last digit removed.
%C A257849 See A142150(n-1) for the base 2 analog, and A115273, A257844 - A257850 for the base 3 - base 10 variants.
%H A257849 Colin Barker, <a href="/A257849/b257849.txt">Table of n, a(n) for n = 0..1000</a>
%H A257849 <a href="/index/Rec#order_18">Index entries for linear recurrences with constant coefficients</a>, signature (0,0,0,0,0,0,0,0,2,0,0,0,0,0,0,0,0,-1).
%F A257849 a(n) = 2*a(n-9)-a(n-18). - _Colin Barker_, May 11 2015
%F A257849 G.f.: x^10*(8*x^7+7*x^6+6*x^5+5*x^4+4*x^3+3*x^2+2*x+1) / ((x-1)^2*(x^2+x+1)^2*(x^6+x^3+1)^2). - _Colin Barker_, May 11 2015
%t A257849 Table[Floor[n/9] Mod[n, 9], {n, 100}] (* _Vincenzo Librandi_, May 11 2015 *)
%o A257849 (PARI) A257849(n)=n\9*(n%9)
%o A257849 (Magma) [Floor(n/9)*(n mod 9): n in [0..100]]; // _Vincenzo Librandi_, May 11 2015
%o A257849 (Sage) [floor(n/9)*(n % 9)  for n in (0..80)]; # _Bruno Berselli_, May 11 2015
%o A257849 (PARI) concat([0,0,0,0,0,0,0,0,0,0], Vec(x^10*(8*x^7+7*x^6+6*x^5+5*x^4+4*x^3+3*x^2+2*x+1) / ((x-1)^2*(x^2+x+1)^2*(x^6+x^3+1)^2) + O(x^100))) \\ _Colin Barker_, May 11 2015
%o A257849 (Python)
%o A257849 from math import prod
%o A257849 def A257849(n): return prod(divmod(n,9)) # _Chai Wah Wu_, Jan 19 2023
%Y A257849 Cf. A142150 (the base 2 analog), A115273, A257844 - A257850.
%K A257849 nonn,base,easy
%O A257849 0,12
%A A257849 _M. F. Hasler_, May 10 2015