

A059097


Numbers n such that the binomial coefficient C(2n,n) is not divisible by the square of an odd prime.


5



0, 1, 2, 3, 4, 6, 7, 9, 10, 11, 12, 21, 22, 28, 29, 30, 31, 36, 37, 54, 55, 57, 58, 110, 171, 784, 786
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OFFSET

1,3


COMMENTS

Probably finite. Erdős believed there were no more terms.
The power of p that divides n! is (ns)/(p1), where s is the sum of the "digits" in the basep representation of n (see Gouvea).  N. J. A. Sloane, Nov 26 2005
From David W. Wilson, Nov 21 2005: (Start)
"The exponent e of prime p in n! can be computed as follows (in pseudoC):
int e(int p, int n) { int s = 0; while (n > 0) { n /= p; s += n; } return s; }
The exponent of p in C(2n, n) is then f(p, n) = e(p, 2*n)  2*e(p, n).
We can then use the following function to check if n is in the sequence:
bool isGood(int n) { for (int p = 3; p <= n; p += 2) { if (!isPrime(p)) continue; if (f(p, n) >= 2) return false; } return true; }" (End)
There are no more terms: Granville and Ramaré show that if n >= 2082 then C(2n,n) is divisible by the square of a prime >= sqrt(n/5).  Robert Israel, Sep 18 2017


REFERENCES

F. Q. Gouvea, pAdic Numbers, SpringerVerlag, 1993; see p. 100.
R. L. Graham, D. E. Knuth and O. Patashnik, Concrete Mathematics, first ed, chap 5, prob 96.
R. K. Guy, Unsolved problems in Number Theory, section B33.


LINKS

Table of n, a(n) for n=1..27.
A. Granville and O. Ramaré, Explicit bounds on exponential sums and the scarcity of squarefree binomial coefficients, Mathematika 43 (1996), 73107, [DOI].


EXAMPLE

C(12,6)=924, which is not divisible by the square of an odd prime, so 6 is in the sequence.


MAPLE

filter:= proc(n)
andmap(t > t[1] = 2 or t[2] = 1, ifactors(binomial(2*n, n))[2]) end proc:
select(filter, [$0..2081]); # Robert Israel, Sep 18 2017


MATHEMATICA

l = {}; Do[m = Binomial[2n, n]; While[EvenQ[m], m = m/2]; If[MoebiusMu[m] != 0, l = {l, n}], {n, 1000}]; Flatten[l] (* Alonso del Arte, Nov 11 2005 *)
(* The following is an implementation of David W. Wilson's algorithm: *)
expoPF[p_, n_] := Module[{s, x}, x = n; s = 0; While[x > 0, x = Floor[x/p]; s = s + x]; s]
expoP2nCn[p_, n_] := expoPF[p, 2*n]  2*expoPF[p, n]
goodQ[ n_ ] := TrueQ[ Module[ {flag, i}, flag = True; i = 2; While[ flag && Prime[ i ] < n, If[ expoP2nCn[ Prime[ i ], n ] > 1, flag = False ]; i++ ]; flag ] ]
Select[ Range[ 1000 ], goodQ[ # ] & ] (* Alonso del Arte, Nov 11 2005 *)


CROSSREFS

Cf. A000984. Cf. also A081391, n such that C[2n, n] has only one prime divisor of which exponent equals one.
Sequence in context: A047519 A070118 A070124 * A047301 A186275 A214857
Adjacent sequences: A059094 A059095 A059096 * A059098 A059099 A059100


KEYWORD

nonn,fini,full,changed


AUTHOR

Jud McCranie, Jan 01 2001


EXTENSIONS

No other terms for n<=157430.  Naohiro Nomoto, May 12 2002
No other terms for n<=2^31  1.  Jack Brennen, Nov 21 2005


STATUS

approved



