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 A217445 Numbers n such that n! has the same number of terminating zeros in bases 3 and 4. 3
 1, 2, 4, 5, 6, 7, 10, 11, 12, 13, 14, 18, 19, 21, 22, 23, 33, 36, 37, 38, 42, 43, 46, 47, 51, 56, 58, 59, 60, 61, 62, 75, 86, 88, 89, 92, 100, 101, 102, 103, 105, 112, 113, 114, 115, 120, 121, 122, 124, 125, 138, 139, 141, 147, 153, 159, 164, 166, 167, 168 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS The number of zeros of n! base 3 is approaching n/2 as n grows. Similarly, the number of zeros of n! base 4 is approaching n/2 as n grows. Consequently, this sequence is expected to have high density. From Robert Israel, Jan 19 2017: (Start) Numbers n such that A000120(n) + (n + A000120(n) mod 2) = A053735(n). Since typically A000120(n) ~ log_2(n) while typically A053735(n) ~ log_3(n), the density of this sequence should go to 0, contrary to the previous comment. (End) LINKS Alois P. Heinz, Table of n, a(n) for n = 1..10000 EXAMPLE 6! is 222200 in base 3 and 23100 in base 4, both of them have 2 zeros at the end, so 6 is in the sequence. MAPLE s2:= n -> convert(convert(n, base, 2), `+`): s3:= n -> convert(convert(n, base, 3), `+`): select(n -> s2(n) + (n+s2(n) mod 2) = s3(n), [\$1..1000]); # Robert Israel, Jan 19 2017 PROG (PARI) is(n)=my(L=log(n+1)); sum(k=1, L\log(3), n\3^k)==sum(k=1, L\log(2), n>>k)\2 \\ Charles R Greathouse IV, Oct 04 2012 CROSSREFS Cf. A054861 (base 3), A090616 (base 4), A090622. Sequence in context: A260820 A248554 A098166 * A007238 A083875 A165291 Adjacent sequences:  A217442 A217443 A217444 * A217446 A217447 A217448 KEYWORD nonn,base AUTHOR Tanya Khovanova, Oct 03 2012 EXTENSIONS More terms from Alois P. Heinz, Oct 03 2012 STATUS approved

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Last modified May 23 07:08 EDT 2019. Contains 323508 sequences. (Running on oeis4.)