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A345217
a(n) is the first emirp that starts a chain of exactly n emirps under the mapping f(x) = x + reverse(x) + 1.
0
13, 149, 70949, 12314597
OFFSET
1,1
COMMENTS
a(n) == 5 (mod 6) for n >= 2.
No more terms < 16*10^11. - Lars Blomberg, Jun 25 2021
EXAMPLE
a(1) = 13 is the first emirp, but 13+31+1 = 45 is not an emirp.
a(2) = 149 because 149 and 149+941+1 = 1091 are emirps, but 1091+1901+1 = 2993 is not an emirp.
a(3) = 70949 because 70949, 70949+94907+1 = 165857, and 165857+758561 = 924419 are emirps, but 924419+914429+1 = 1838849 is not an emirp.
MAPLE
revdigs:= proc(n) local L, i;
L:= convert(n, base, 10);
add(L[-i]*10^(i-1), i=1..nops(L))
end proc:
f:= proc(n) local r, t, x;
x:= n;
for t from 0 do
if not isprime(x) then return t fi;
r:= revdigs(x);
if not (r <> n and isprime(x) and isprime(r)) then return t fi;
x:= x+r+1;
od;
end proc:
V:= Vector(4): V[1]:= 13: count:= 1:
for n from 17 by 6 while count < 4 do
v:= f(n);
if v > 0 and V[v] = 0 then V[v]:= n; count:= count+1 fi;
od:
convert(V, list);
CROSSREFS
Cf. A006567.
Sequence in context: A125448 A163148 A355414 * A185405 A051475 A242576
KEYWORD
nonn,base,more
AUTHOR
J. M. Bergot and Robert Israel, Jun 10 2021
STATUS
approved