OFFSET
0,1
COMMENTS
Proposition: a(n) ends with exactly 2^n 9's.
The number of digits in a(n) is respectively 1, 5, 18, 73, 289, 1156, 4622, ... At each step until a(6), the number of digits of a(n+1) is nearly 4 times the number of digits of a(n), and exactly so from a(4) to a(5).
Sequence has doubly-exponential growth: log log a(n) ~ n. - Charles R Greathouse IV, Mar 27 2020
REFERENCES
Eric Billault, Walter Damin, Robert Ferréol et al., MPSI - Classes Prépas, Khôlles de Maths, Ellipses, 2012, exercice 4.21 pages 79 and 91.
EXAMPLE
a(2) = 3*22599^4 + 4*22599^3 = 782534990456559999 ends with 2^2 digits 9's.
MATHEMATICA
a[0] = 9; a[n_] := a[n] = 3*a[n-1]^4 + 4*a[n-1]^3; Array[a, 4, 0] (* Amiram Eldar, Mar 26 2020 *)
CROSSREFS
KEYWORD
nonn,base
AUTHOR
Bernard Schott, Mar 26 2020
STATUS
approved