A378054 - OEIS

A378054

Numbers k that divide A378053(k) = gcd(Product_{d|k} (d + 1), Product_{d|k, d>1} (d - 1)).

1, 60, 90, 120, 144, 168, 180, 210, 240, 252, 280, 336, 360, 420, 504, 540, 560, 630, 660, 720, 840, 900, 924, 990, 1008, 1056, 1080, 1092, 1200, 1260, 1320, 1404, 1440, 1512, 1560, 1680, 1800, 1848, 1872, 1890, 1980, 2016, 2100, 2112, 2160, 2184, 2310, 2376, 2400

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,2

COMMENTS

After the first term a(1) = 1, the next odd term is a(71) = 3465, the next term that is coprime to 6 is a(1058) = 95095, and the next term that is coprime to 30 is a(12174) = 2263261.

LINKS

Amiram Eldar, Table of n, a(n) for n = 1..10000

EXAMPLE

60 is a term since A378053(60) = 166320 = 60 * 2772 is divisible by 60.

MATHEMATICA

Select[Range[2500], And @@ Divisible[{Times @@ ((d = Divisors[#]) + 1), Times @@ (Rest @ d - 1)}, #] &]

PROG

(PARI) is(k) = if(k == 1, 1, my(d = divisors(k)); !(gcd(prod(k=1, #d, d[k]+1), prod(k=2, #d, d[k]-1)) % k));

CROSSREFS

Intersection of A056954 and A355331.

A378055 is a subsequence.

Cf. A020696, A377484, A378053, A378058.