OFFSET
0,3
COMMENTS
Eswaran observes that n is divisible by 7 iff repeated application of a ends at the number 7.
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
REFERENCES
R. Eswaran, Test of divisibility of the number 7, Abstracts Amer. Math. Soc., 23 (No. 2, 2002), #974-00-5, p. 275.
LINKS
Index entries for linear recurrences with constant coefficients, signature (1,0,0,0,0,0,0,0,0,1,-1).
FORMULA
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
a(n) = n-7*floor(n/10). - Wesley Ivan Hurt, May 12 2016
MAPLE
A081502 := proc(n)
local x, y ;
y := modp(n, 10) ;
x := iquo(n, 10) ;
3*x+y ;
end proc:
seq(A081502(n), n=0..120) ; # R. J. Mathar, Oct 03 2014
MATHEMATICA
Table[n - 7 * Floor[n / 10], {n, 0, 100}] (* Joshua Oliver, Dec 04 2019 *)
PROG
(PARI) a(n) = 3*(n\10) + (n % 10); \\ Michel Marcus, Mar 19 2014
(PARI) a(n) = [3, 1]*divrem(n, 10); \\ Kevin Ryde, Dec 04 2019
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
N. J. A. Sloane, Apr 22 2003
STATUS
approved