A192988 - OEIS

%I #15 Nov 30 2019 09:14:32

%S 1,2,3,4,9,21,91,1517,385318,24745571498,102057218155974827415,

%T 1735945962956039658808082636319818286550

%N Smallest number whose representation requires n pentagonal numbers with greedy algorithm.

%F a(n+1) = ceiling(a(n)/3)*(3*ceiling(a(n)/3)-1)/2 + a(n), a(1) = 1.

%e a(7)=91 since 91 = 70 + 12 + 5 + 1 + 1 + 1 + 1.

%t a[1] = 1; a[n_] := a[n] = Ceiling[a[n - 1]/3]*(3*Ceiling[a[n - 1]/3] - 1)/2 + a[n - 1]; Table[a[n], {n, 15}] (* _T. D. Noe_, Jul 13 2011 *)

%Y Cf. A145172.

%K nonn

%O 1,2

%A _Franz Vrabec_, Jul 13 2011