OFFSET
2,2
FORMULA
For n>10, a(n) is nonzero only if n-1 divides k*10^s for some positive integers k<=9 and s. For minimum such number k*10^s, we have a(n) = n*k*10^s/(n-1). - Max Alekseyev, Apr 13 2009
EXAMPLE
a(7) = 35: 35/7 = 5 = 35 with leftmost digit deleted.
PROG
(PARI) { a(n) = if(n<=10, return([0, 0, 15, 0, 25, 12, 35, 0, 45, 0][n])); t=[valuation(n-1, 2), valuation(n-1, 5)]; k=(n-1)\2^t[1]\5^t[2]; if(k>9, return(0)); while(t[1]>t[2]&&2*k<=9, k*=2; t[1]--); while(t[1]<t[2]&&5*k<=9, k*=5; t[2]--); (k*10^vecmax(t)*n)\(n-1) } \\ Max Alekseyev, Apr 13 2009
CROSSREFS
KEYWORD
nonn,base
AUTHOR
Lekraj Beedassy, Jul 29 2008
EXTENSIONS
Corrected and extended by Max Alekseyev, Apr 13 2009
STATUS
approved