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A162729
a(n) = 5^n*(5^n-1)/2.
2
0, 10, 300, 7750, 195000, 4881250, 122062500, 3051718750, 76293750000, 1907347656250, 47683710937500, 1192092871093750, 29802322265625000, 745058059082031250, 18626451489257812500, 465661287292480468750
OFFSET
0,2
FORMULA
a(n) = A000217(A000351(n)-1) = 30*a(n-1)-125*a(n-2). G.f.: 10*x/((25*x-1)*(5*x-1)). - R. J. Mathar, Jul 13 2009
MATHEMATICA
n5[n_]:=Module[{c=5^n}, (c(c-1))/2]; Array[n5, 20, 0] (* or *) LinearRecurrence[ {30, -125}, {0, 10}, 20] (* Harvey P. Dale, Oct 25 2011 *)
PROG
(Magma) [5^n*(5^n-1)/2: n in [0..20]]; // Vincenzo Librandi, Mar 05 2012
CROSSREFS
Sequence in context: A173478 A069672 A129005 * A278037 A275652 A221953
KEYWORD
nonn,easy
AUTHOR
Vincenzo Librandi, Jul 12 2009
EXTENSIONS
Edited by R. J. Mathar, Jul 13 2009
STATUS
approved