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A081502 Let n = 10x + y where 0 <= y <= 9, x >= 0. Then a(n) = 3x+y.

%I A081502 #28 Dec 05 2019 04:42:00
%S A081502 0,1,2,3,4,5,6,7,8,9,3,4,5,6,7,8,9,10,11,12,6,7,8,9,10,11,12,13,14,15,
%T A081502 9,10,11,12,13,14,15,16,17,18,12,13,14,15,16,17,18,19,20,21,15,16,17,
%U A081502 18,19,20,21,22,23,24,18,19,20,21,22,23,24,25,26,27,21,22,23,24,25,26,27,28,29
%N A081502 Let n = 10x + y where 0 <= y <= 9, x >= 0. Then a(n) = 3x+y.
%C A081502 Eswaran observes that n is divisible by 7 iff repeated application of a ends at the number 7.
%C A081502 a(n) is divisible by 7 iff n is divisible by 7: e.g., a(7) = a(14) = a(21) = 7, a(28) = a(35) = a(42) = 14 etc. - _Zak Seidov_, Mar 19 2014
%D A081502 R. Eswaran, Test of divisibility of the number 7, Abstracts Amer. Math. Soc., 23 (No. 2, 2002), #974-00-5, p. 275.
%H A081502 <a href="/index/Rec#order_11">Index entries for linear recurrences with constant coefficients</a>, signature (1,0,0,0,0,0,0,0,0,1,-1).
%F A081502 G.f.: -x*(6*x^9-x^8-x^7-x^6-x^5-x^4-x^3-x^2-x-1) / (x^11-x^10-x+1). - _Colin Barker_, Mar 19 2014
%F A081502 a(n) = n-7*floor(n/10). - _Wesley Ivan Hurt_, May 12 2016
%p A081502 A081502 := proc(n)
%p A081502     local x,y ;
%p A081502     y := modp(n,10) ;
%p A081502     x := iquo(n,10) ;
%p A081502     3*x+y ;
%p A081502 end proc:
%p A081502 seq(A081502(n),n=0..120) ; # _R. J. Mathar_, Oct 03 2014
%t A081502 Table[n - 7 * Floor[n / 10], {n, 0, 100}] (* _Joshua Oliver_, Dec 04 2019 *)
%o A081502 (PARI) a(n) = 3*(n\10) + (n % 10); \\ _Michel Marcus_, Mar 19 2014
%o A081502 (PARI) a(n) = [3,1]*divrem(n,10); \\ _Kevin Ryde_, Dec 04 2019
%Y A081502 Different from A028898 for n>=100 (e.g. a(111) = 34, A029989(111) = 13).
%Y A081502 Cf. A081503, A081594, A081595, A081596, A081597, A081598, A081599, A081600.
%K A081502 nonn,easy
%O A081502 0,3
%A A081502 _N. J. A. Sloane_, Apr 22 2003