OFFSET
0,5
COMMENTS
Conjecture: For n>3 a(n) is positive.
For 4 <= n <= 102, a(n) is the product of two distinct primes, but a(103) = a(49)*a(54) and is the product of four distinct primes: 1862645149230957031249999 * 5368709119999999999999999 * 79999999999999999999999999 * 12499999999999999999999999999. - David Wasserman, Nov 18 2008
LINKS
Max Alekseyev, Table of n, a(n) for n = 0..1000
Max A. Alekseyev, Computing the Inverses, their Power Sums, and Extrema for Euler's Totient and Other Multiplicative Functions. Journal of Integer Sequences, Vol. 19 (2016), Article 16.5.2
EXAMPLE
a(12)=999997950001 because sigma(999997950001)=sigma(799999*1249999) =800000*1250000=10^12 and 999997950001 is the largest number with this property(sigma(m)=10^12).
MATHEMATICA
a[0] = 1; a[1] = a[2] = a[3] = 0; a[n_] := (For[m = 1, DivisorSigma[ 1, 10^n - m] != 10^n, m++ ]; 10^n - m); Do[Print[a[n]], {n, 0, 12}]
CROSSREFS
KEYWORD
nonn
AUTHOR
Farideh Firoozbakht, Jul 31 2005
EXTENSIONS
More terms from David Wasserman, Nov 18 2008
Terms a(19) onward from Max Alekseyev, Mar 06 2014
STATUS
approved