A253137 a(n) = n*gcd(c(n), d(n)), where c(n) = composite numbers, d(n) = number of divisors of c(n).

1, 4, 12, 12, 10, 36, 14, 8, 9, 60, 22, 12, 26, 112, 15, 32, 17, 36, 38, 40, 21, 44, 23, 216, 50, 26, 216, 56, 58, 90, 62, 64, 33, 68, 35, 72, 74, 38, 312, 40, 82, 504, 86, 132, 45, 46, 94, 96, 49, 100, 612, 104, 159, 108, 55, 112, 570, 58, 118, 720, 61, 124, 63, 512, 390, 66, 134, 68, 138, 70, 852, 144, 219, 74, 150, 608, 77, 156, 948

Offset

1,2

Comments

Occurrences of primes: 3 (17, 23, 61) in first 80 terms.

Links

Table of n, a(n) for n=1..79.

Example

For n = 4: 4th composite number is 9. 9 has 3 divisors (1,3,9), thus gcd(9,3) * 4 = 12.

Mathematica

Composites := Select[Range[2, 110], ! PrimeQ[#] &]; Composite[n_] := Last[Take[Composites, n]]; Table[GCD[Composite[n], DivisorSigma[0, Composite[n]]] n, {n, Length[Composites]}]

Crossrefs

Cf. A002808, A000005.

Sequence in context: A238581 A063608 A074258 * A378187 A120213 A005886

Adjacent sequences: A253134 A253135 A253136 * A253138 A253139 A253140

Keyword

nonn,easy

Author

Carlos Eduardo Olivieri, Dec 27 2014

Status

approved