OFFSET
1,1
COMMENTS
Values of the primes corresponding to A073805, sorted and with duplicates removed.
Most terms can be obtained in two ways, corresponding to x * R(x) + 1 and R(x) * x + 1 or more generally (10^i * x) * R(x) + 1 and (10^i * R(x)) * x + 1, where R(x) <> x and x doesn't end in 0 so R(R(x)) = x. The first term that can be obtained in four ways is 1015561 = 1560 * 651 + 1 = 2730 * 372 + 1 = 3720 * 273 + 1 = 6510 * 156 + 1.
LINKS
Robert Israel, Table of n, a(n) for n = 1..10000
EXAMPLE
a(1) = 2 = 1 * 1 + 1.
a(3) = 11 = 10 * 1 + 1.
a(13) = 977 = 16 * 61 + 1.
MAPLE
N:= 6: # for terms <= 10^N where N is even
S:= {}:
for x from 1 to 10^(N/2)-1 do
if x mod 10 = 0 then next fi;
r:= rev(x);
if r < x then next fi;
v:= x*r;
for i from 0 do
w:= 10^i*v+1;
if w > 10^N then break fi;
if isprime(w) then S:= S union {w} fi;
od
od:
sort(convert(S, list));
CROSSREFS
KEYWORD
nonn,base
AUTHOR
Robert Israel, Jul 03 2024
STATUS
approved