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A248868
Exponents n that make k! < k^n < (k+1)! hold true for some integer k > 1, in increasing order by k, then n (if applicable).
1
2, 2, 3, 3, 4, 4, 5, 6, 6, 7, 8, 9, 9, 10, 11, 12, 12, 13, 14, 15, 15, 16, 17, 18, 19, 19, 20, 21, 22, 22, 23, 24, 25, 26, 26, 27, 28, 29, 30, 30, 31, 32, 33, 34, 34, 35, 36, 37, 38, 38, 39, 40, 41, 42, 43, 43, 44, 45, 46, 47, 47, 48, 49, 50, 51, 51, 52, 53, 54
OFFSET
1,1
COMMENTS
This sequence consists of those positive integers that, when taken as exponents of some positive integer greater than 1, make the corresponding power of that other integer fall strictly between its factorial and the factorial of the next integer, as shown in the examples.
The sequence { floor(log_n((n+1)!)) | n>=2 } is a subsequence.
This sequence is nondecreasing. Indeed for k>1, k^n<(k+1)! implies n<=k, which implies ((k+1)/k)^(n-1) <= (1 + 1/k)^(k-1) = Sum_{i=0..k-1} binomial(k-1,i) (1/k)^i < Sum_{i=0..k-1} ((k-1)/k)^i < k, which implies (k+1)^(n-1)<k^n<(k+1)!. - Danny Rorabaugh, Apr 03 2015
From Danny Rorabaugh, Apr 15 2015: (Start)
This sequence is the same as A074184 for 6<=n<=10000.
For k > 2, k! < k^(ceiling(log_k(k!))) < (k+1)!.
The two sequences continue to be identical provided k^(1 + ceiling(log_k(k!))) > (k+1)! when k > 5.
This is equivalent to k^(2 - fractional_part(log_k(k!))) > k + 1, which can be approximated by fractional_part(1/2 - (k + sqrt(2*Pi))/log(k)) < 1 - 1/(k*log(k)) using Stirling's approximation.
Are either of the final inequalities true for all sufficiently large k?
(End)
LINKS
EXAMPLE
2! < 2^2 < 3! < 3^2 < 4! < 4^3 < 5! < 5^3 < 5^4 < 6! < 6^4 < 7! < 7^5 < 8! and so on; this sequence consists of the exponents.
PROG
(Sage)
[x for sublist in [[k for k in [0..ceil(log(factorial(n+1), base=n))] if (factorial(n)<n^k and n^k<factorial(n+1))] for n in [2..100]] for x in sublist] # Tom Edgar, Mar 04 2015
CROSSREFS
KEYWORD
nonn
AUTHOR
Juan Castaneda, Mar 04 2015
EXTENSIONS
More terms from Tom Edgar, Mar 04 2015
STATUS
approved