OFFSET
1,1
COMMENTS
a(13) <= 102564102564102564. - Donovan Johnson, Jun 06 2009
The condition is equivalent to constraining the numbers to be of the form 10*m+d with a k-digit number m and a nonzero digit d such that 4*(10*m+d) = 10^k * d + m, i.e., 39*m = (10^k - 4)*d. Checking modulo 13, this implies k = 5 (mod 6). Also, m >= 10^(k-1) implies d >= 4. Each such k and d leads to a solution. - Hagen von Eitzen, Jun 26 2009
LINKS
Wikipedia, Parasitic number.
FORMULA
If n = 6*k + r with 1 <= r <= 6, then a(n) = (10^(6*k) - 1)/(10^6 - 1)*a(r) as well as a(n) = floor((r + 3)/39*10^(6*(k+1))). - Hagen von Eitzen, Jun 26 2009
PROG
(PARI) a(n) = local(r=(n-1)%6+1, k=(n-r)/6); floor((r+3)/39*10^(6*(k+1))) \\ Hagen von Eitzen, Jun 26 2009
CROSSREFS
KEYWORD
nonn,base
AUTHOR
N. J. A. Sloane, based on correspondence from William A. Hoffman III (whoff(AT)robill.com), Apr 10 2009
EXTENSIONS
a(7)-a(12) from Donovan Johnson, Jun 06 2009
More terms from Hagen von Eitzen, Jun 26 2009
STATUS
approved