A263152 a(n) is the greatest common unitary divisor of the friendly pairs, A050972(n) and A050973(n).

1, 5, 1, 1, 1, 1, 11, 13, 17, 1, 1, 19, 3, 23, 3, 25, 29, 1, 31, 1, 37, 41, 5, 43, 47, 7, 53, 3, 1, 55, 7, 2, 1, 59, 61, 9, 65, 67, 71, 9, 73, 11, 79, 83, 85, 11, 5, 5, 89, 11, 13, 95, 97, 101, 103, 13, 11, 107, 109, 113, 115, 4, 121, 17, 7, 125, 13, 127, 131

Offset

1,2

Comments

Dividing both A050972(n) and A050973(n) by a "greater than 1" divisor of a(n), if any, will give a smaller friendly pair.

If a(n) is greater than 1, dividing both A050972(n) and A050973(n) will give a primitive friendly pair.

Links

Michel Marcus, Table of n, a(n) for n = 1..10000

Formula

a(n) = A165430(A050972(n), A050973(n)).

a(A263118(n)) = 1, the primitive friendly pairs.

Example

The greatest common unitary divisor of the first friendly pair (6, 28) is 1, hence a(1) = 1.

Prog

(PARI) udivs(n) = {my(d = divisors(n)); select(x->(gcd(x, n/x)==1), d); }
ugcd(x, y) = vecmax(setintersect(udivs(x), udivs(y)));
lista(vp, vg) = {for (n=1, #vp, print1(ugcd(vp[n], vg[n]), ", ")); ); } \\ where vp and vg are A050972 and A050973

Crossrefs

Cf. A050972, A050973, A263118.

Cf. A165430 (greatest common unitary divisor).

Sequence in context: A358347 A367989 A220054 * A075463 A026518 A362394

Adjacent sequences: A263149 A263150 A263151 * A263153 A263154 A263155

Keyword

nonn

Author

Michel Marcus, Oct 11 2015

Status

approved