A088830 a(n) = Min{x : sigma(x) = n*phi(x), x is not a prime}, the least nonprime solutions to sigma(x) = n*phi(x); special balanced numbers.

1, 35, 15, 14, 56, 6, 12, 42, 30, 168, 2580, 210, 630, 420, 840, 20790, 416640, 9240, 291060, 83160, 120120, 5165160, 1719277560, 43825320, 26860680, 277560360, 1304863560, 569729160, 587133466920, 16522145640, 33044291280, 563462139240, 1140028049160

Offset

1,2

Comments

a(33) > 10^12. - Donovan Johnson, Sep 03 2013

a(34) <= 9015394227840, a(35) <= 1255683068640. - Giovanni Resta, May 08 2017

Links

Amiram Eldar, Table of n, a(n) for n = 1..36 (calculated using data from Jud McCranie)

Formula

For n > 3, a(n) = A055234(n). - David Wasserman, Aug 18 2005

Mathematica

ds[x_, de_] := DivisorSigma[1, x]-de*EulerPhi[x] a[n_] := Block[{m=1, s=ds[m, n]}, While[(s !=0||PrimeQ[m])&&!Greater[m, 100000], m++ ]; m]; Table[a[n], {n, 22}]

Crossrefs

Compare A087979, which has a slightly different definition.

Cf. A074891, A000203, A000010, A087979, A020492.

Cf. A055234.

Sequence in context: A267198 A267140 A348893 * A033355 A267402 A217408

Adjacent sequences: A088827 A088828 A088829 * A088831 A088832 A088833

Keyword

nonn

Author

Labos Elemer, Nov 03 2003

Extensions

More terms from David Wasserman, Aug 18 2005

a(32) from Donovan Johnson, Sep 03 2013

a(33) from Giovanni Resta, May 08 2017

Status

approved