A130767 a(n) = product{k=1 to n} (k-th integer from among those positive integers which are coprime to (n+1-k)).

1, 2, 9, 40, 420, 2700, 56595, 419328, 8820900, 88488400, 2327925600, 38767286880, 1912404574080, 21612951360000, 644047087612500, 10985391056640000, 634391869996684800, 14046187624838328960, 764077915447610400000, 15840110879873280000000, 755098009918296312668400

Offset

1,2

Comments

a(n) is the product of the terms in the n-th antidiagonal of the A126572 array. - Michel Marcus, Mar 14 2018

Links

Table of n, a(n) for n=1..21.

Example

The integers coprime to 1 are: 1,2,3,4,5,6,... The 5th of these is 5. The integers coprime to 2 are: 1,3,5,7,9,... The 4th of these is 7. The integers coprime to 3 are: 1,2,4,5,7,... The 3rd of these is 4. The integers coprime to 4 are: 1,3,5,... The 2nd of these is 3. And the integers coprime to 5 are: 1,2,3,4,6,... The first of these is 1. So a(5) = 5 * 7 * 4 * 3 * 1 = 420.

Prog

(PARI) cop(k, j) = {my(nbc = 0, i = 0); while (nbc != j, i++; if (gcd(i, k)==1, nbc++)); i; }
a(n) = {my(vc = vector(n, k, cop(k, n-k+1))); prod(k=1, n, vc[k]); } \\ Michel Marcus, Mar 14 2018

Crossrefs

Cf. A126572, A132273, A132421.

Sequence in context: A259339 A346841 A052322 * A273461 A217190 A020698

Adjacent sequences: A130764 A130765 A130766 * A130768 A130769 A130770

Keyword

nonn

Author

Leroy Quet, Aug 18 2007

Extensions

More terms from Michel Marcus, Mar 14 2018

Status

approved