OFFSET
0,1
COMMENTS
This is the 10's complement of A225405.
LINKS
Seiichi Manyama, Table of n, a(n) for n = 0..10000
FORMULA
Define the sequence {b(n)} by the recurrence b(0) = 0 and b(1) = 7, b(n) = b(n-1) + 7 * (b(n-1)^3 + 7) mod 10^n for n > 1, then a(n) = (b(n+1) - b(n))/10^n. - Seiichi Manyama, Aug 13 2019
EXAMPLE
7^3 == -7 (mod 10).
57^3 == -7 (mod 10^2).
457^3 == -7 (mod 10^3).
8457^3 == -7 (mod 10^4).
48457^3 == -7 (mod 10^5).
948457^3 == -7 (mod 10^6).
PROG
(PARI) n=0; for(i=1, 100, m=(10^i-7); for(x=0, 9, if(((n+(x*10^(i-1)))^3)%(10^i)==m, n=n+(x*10^(i-1)); print1(x", "); break)))
(PARI) N=100; Vecrev(digits(lift(chinese(Mod((-7+O(2^N))^(1/3), 2^N), Mod((-7+O(5^N))^(1/3), 5^N)))), N) \\ Seiichi Manyama, Aug 04 2019
(Ruby)
def A225408(n)
ary = [7]
a = 7
n.times{|i|
b = (a + 7 * (a ** 3 + 7)) % (10 ** (i + 2))
ary << (b - a) / (10 ** (i + 1))
a = b
}
ary
end
p A225408(100) # Seiichi Manyama, Aug 13 2019
CROSSREFS
KEYWORD
nonn,base
AUTHOR
Aswini Vaidyanathan, May 07 2013
STATUS
approved