login
A263152
a(n) is the greatest common unitary divisor of the friendly pairs, A050972(n) and A050973(n).
1
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
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. A165430 (greatest common unitary divisor).
Sequence in context: A358347 A367989 A220054 * A075463 A026518 A362394
KEYWORD
nonn
AUTHOR
Michel Marcus, Oct 11 2015
STATUS
approved