|
|
A218394
|
|
Numbers such that sum(i<=n) binomial(n,i)*binomial(2*n-2*i, n-i) is not divisible by 5.
|
|
0
|
|
|
1, 5, 7, 11, 25, 27, 31, 35, 37, 51, 55, 57, 61, 125, 127, 131, 135, 137, 151, 155, 157, 161, 175, 177, 181, 185, 187, 251, 255, 257, 261, 275, 277, 281, 285, 287, 301, 305, 307, 311, 625, 627, 631, 635, 637, 651, 655, 657, 661, 675, 677, 681, 685, 687, 751
(list;
graph;
refs;
listen;
history;
text;
internal format)
|
|
|
OFFSET
|
1,2
|
|
COMMENTS
|
a(n) = A037453(2*n-1) (proved by Schur, see link).
|
|
LINKS
|
|
|
FORMULA
|
a(n)=2*n - 1 + 2*sum{i=1,n} 5^(i-1)*floor((2*n-1)/3^i).
|
|
PROG
|
(PARI) lista(nb) = {for (n=1, nb, if (sum(i=1, n, binomial(n, i)*binomial(2*n-2*i, n-i)) % 5 != 0, print1(n, ", ")); ); }
(PARI) a(n) = {2*n-1+2*sum(i=1, n, 5^(i-1)*floor((2*n-1)/3^i))}
|
|
CROSSREFS
|
|
|
KEYWORD
|
nonn
|
|
AUTHOR
|
|
|
STATUS
|
approved
|
|
|
|