A084321 -id:A084321 - cp's OEIS frontend

A085355 a(n) is the difference between A084321(n) and the (n-1)th power of 2.

0, 1, 1, 2, 3, 3, 0, 11, 0, 24, 37, 47, 73, 90, 133, 135, 110, 133, 435, 795, 317, 1674, 1217, 1975, 4816, 6177, 9483, 11885, 17724, 19562, 22958, 39870, 48417, 85452, 114676, 210482, 133092, 194048, 524165, 451743, 1145779, 1292884, 1604772, 2859433, 4032594

Offset: 1

Labos Elemer and Robert G. Wilson v, Jun 24 2003

nonn

A084321 is the least number k such that between k! and (k+1)! there are n powers of 2.

Kevin Ryde, Table of n, a(n) for n = 1..1000

Cf. A084321.

LogBase2Stirling[n_] := Floor[ Log[2, 2 Pi n]/2 + n*Log[2, n/E] + Log[2, 1 + 1/(12n) + 1/(288n^2) - 139/(51840n^3) - 571/(2488320n^4) + 163879/(209018880n^5)]]; Do[k = 2^(n - 1); While[ LogBase2Stirling[k + 1] - LogBase2Stirling[k] < n, k++ ]; a[n] = k, {n, 1, 45}]; Table[ a[n] - 2^(n - 1), {n, 1, 45}]

A085301 Number of factorials between two primorials.

Original entry on oeis.org

2, 2, 1, 1, 1, 1, 2, 1, 1, 2, 1, 1, 1, 2, 1, 1, 2, 1, 1, 2, 1, 1, 2, 1, 1, 1, 2, 1, 1, 2, 1, 1, 2, 1, 1, 1, 2, 1, 1, 2, 1, 1, 2, 1, 1, 1, 2, 1, 1, 2, 1, 1, 2, 1, 1, 1, 2, 1, 1, 2, 1, 1, 2, 1, 1, 1, 2, 1, 1, 2, 1, 1, 2, 1, 1, 1, 2, 1, 1, 2, 1, 1, 1, 2, 1, 1, 2, 1, 1, 2, 1, 1, 1, 2, 1, 1, 2, 1, 1, 1, 2, 1, 1, 2, 1

Offset: 1

Labos Elemer, Jun 26 2003

nonn

Seems provable: a(n) > 0 for all n; seems more difficult to prove (if true at all) that a(n)=1 or 2; for n < 2050 it holds. Stirling's approximation and Prime Number Theorem together may help.

			n=1: between 1st (=2) and 2nd (=6) primorials, the factorials 2!=2 and 3!=6 occur, so a(1)=2.
n=2: between the primorials 6 and 30, the factorials 3!=6 and 4!=24 occur, so a(2)=2.
Factorial and primorial sets coincide only in case of n = 1,2: {2,6}.
If n > 3, factorials are never squarefree; but primorials are always squarefree, so they are disjoint.
n=5: between the 5th and 6th primorials 2310 and 30030, only the factorial 7!=5040 occurs.
n=6: between the primorials 30030 and 510510, the factorials 8!=40320 and 9!=362880 occur.

Amiram Eldar, Table of n, a(n) for n = 1..10000

Cf. A000142, A002110, A067850, A084320, A084321, A084558, A084972, A085355.

fn[n_] := Module[{k = 1, r = n}, While[r >= 1, k++; r /= k]; k - 1];
prim[n_] := Times @@ Prime[Range[n]];
a[n_] := fn[prim[n]] - fn[prim[n - 1]]; a[1] = a[2] = 2; Array[a, 100] (* Amiram Eldar, Oct 24 2024 *)

a(n) = Card[{k; q(n) <= k! <= q(n+1)}, where q(j) = A002110(j), the j-th primorial; closed intervals required only for n = 1, 2.

a(n) = A084558(A002110(n)) - A084558(A002110(n-1)) for n >= 3. - Amiram Eldar, Oct 24 2024

A084420 Least number k such that between k! and (k+1)! there are n powers of ten.

Original entry on oeis.org

3, 14, 103, 1042, 10158, 100502, 1000617, 10006509, 100019088, 1000004377, 10000170793, 100000442970, 1000001981666, 10000005339905, 100000018997256, 1000000065392525, 10000000201014294, 100000000631520633, 1000000001077349719, 10000000005309690910

Offset: 1

Robert G. Wilson v, Jun 26 2003

nonn

Kevin Ryde, Table of n, a(n) for n = 1..400
Kevin Ryde, C code.

Cf. A084321.

C
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LogBase10Stirling[n_] := Floor[ Log[10, 2 Pi n]/2 + n * Log[10, n/E] + Log[10, 1 + 1/(12n) + 1/(288n^2) - 139/(51840n^3) - 571/(2488320n^4) + 163879/(209018880n^5)]]; Do[k = 10^(n - 1); While[ LogBase10Stirling[k + 1] - LogBase10Stirling[k] < n, k++ ]; a[n] = k; Print[ a[n]], {n, 1, 14}]

a(15)-a(20) from Kevin Ryde, Mar 09 2024

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A085355 a(n) is the difference between A084321(n) and the (n-1)th power of 2.

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A085301 Number of factorials between two primorials.

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A084420 Least number k such that between k! and (k+1)! there are n powers of ten.

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