cp's OEIS Frontend

A365458 The largest power of 3 that is less than or equal to n.

%I A365458 #22 Sep 17 2023 18:27:42
%S A365458 1,1,3,3,3,3,3,3,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,9,27,27,27,27,27,
%T A365458 27,27,27,27,27,27,27,27,27,27,27,27,27,27,27,27,27,27,27,27,27,27,27,
%U A365458 27,27,27,27,27,27,27,27,27,27,27,27,27,27,27,27,27,27,27,27,27,27,27,27,27,27,81,81,81,81,81
%N A365458 The largest power of 3 that is less than or equal to n.
%H A365458 Michael De Vlieger, <a href="/A365458/b365458.txt">Table of n, a(n) for n = 1..19683</a> (3^9 = 19683)
%F A365458 a(n) = 3^floor((log n) / (log 3)). - _Michael De Vlieger_, Sep 17 2023
%F A365458 a(n) =  A000244(A062153(n)). - _Michel Marcus_, Sep 17 2023
%e A365458 a(2) = 1 because 3^0 = 1 <= 2.
%e A365458 a(3) = 3 because 3^1 = 3 <= 3.
%e A365458 a(4) = 3 because 3^1 = 3 <= 4.
%t A365458 Array[3^Floor@ Log[3, #] &, 90] (* _Michael De Vlieger_, Sep 17 2023 *)
%o A365458 (PARI) A365458(n) = if(1==n,n,my(k=0); while((3^k) < n, k++); if((3^k) > n,k--); (3^k));
%o A365458 (PARI) a(n) = 3^logint(n, 3); \\ _Michel Marcus_, Sep 17 2023
%o A365458 (Python)
%o A365458 def A365458(n):
%o A365458     kmin, kmax = 0, 1
%o A365458     while 3**kmax <= n:
%o A365458         kmax <<= 1
%o A365458     while True:
%o A365458         kmid = kmax+kmin>>1
%o A365458         if 3**kmid > n:
%o A365458             kmax = kmid
%o A365458         else:
%o A365458             kmin = kmid
%o A365458         if kmax-kmin <= 1:
%o A365458             break
%o A365458     return 3**kmin # _Chai Wah Wu_, Sep 17 2023
%Y A365458 Cf. A000244, A365459.
%Y A365458 Cf. also A062153, A064235, A080342.
%K A365458 nonn
%O A365458 1,3
%A A365458 _Antti Karttunen_, Sep 17 2023