OFFSET
1,1
COMMENTS
a(4)=156521739130434782608695652173913043478260869565217391304348.
From Robert Israel, Aug 24 2023: (Start)
If 9 * 10^d + 1 = a^2 * b with a > 1, then a * b * c is a term if a^2/(90 + 10^(1-d)) < c^2 < a^2/(9 + 10^(-d)). For example, 9 * 10^d + 1 is divisible by 7^2 for d == 37 (mod 42), and then (9 * 10^d + 1)/7 and 2*(9 * 10^d + 1)/7 are terms. In particular, the sequence is infinite. (End)
LINKS
Robert Israel, Table of n, a(n) for n = 1..12
MAPLE
F:= proc(d) local R, F, t, b, r, q, s, m0, x0, k;
R:= NULL;
F:= ifactors(9*10^d+1)[2];
b:= mul(t[1]^floor(t[2]/2), t=F);
for r in numtheory:-divisors(b) do
x0:= (9*10^d+1)/r;
m0:= x0/r;
for k from ceil(sqrt(10^(d-1)/m0)) to floor(sqrt(10^d/m0)) do
R:= R, x0*k;
od
od;
R
end proc:
sort(map(F, [$1..90])); # Robert Israel, Aug 24 2023
CROSSREFS
KEYWORD
nonn,base,bref
AUTHOR
Giovanni Resta, Jan 25 2006
EXTENSIONS
Definition modified by Georg Fischer, Jul 26 2019
STATUS
approved