OFFSET
1,1
LINKS
Robert Israel, Table of n, a(n) for n = 1..1600
MAPLE
revdigs:= proc(x) local F, i;
F:= convert(x, base, 10);
add(F[-i]*10^(i-1), i=1..nops(F))
end proc:
f1:= proc(n)
local m0, a0, b0, m, a, b, c, x;
m0:= ilog10(n)+1;
if m0::even then m:= m0/2+1; a0:= 1; b0:= 0;
else a0:= floor(n/10^(m0-1));
if a0 = 4 or a0 = 5 then a0:= 7; b0:= 0
elif a0::odd then b0:= n - 10^(m0-1)*a0;
else a0:= a0+1; b0:= 0;
fi;
m:= ceil(m0/2); b0:= floor(b0/10^(m-1));
fi;
for a from a0 to 9 by 2 do
for b from b0 to 10^(m-1) do
x:= 10^(m-1)*a + b;
x:= 10^(m-1)*x + revdigs(floor(x/10));
if x < n then next fi;
if isprime(x) then return x fi
od;
b0:= 0;
od;
procname(10^m0);
end proc;
A[1]:= 2: A[2]:= 5: A[3]:= 11:
for n from 4 to 30 do
A[n]:= f1(2*A[n-1]-A[n-2]+1);
od:
seq(A[i], i=1..30); # Robert Israel, Jan 31 2019
MATHEMATICA
p = 1; d = 0; Do[ q = FromDigits[ Join[ IntegerDigits[n], Drop[ Reverse[ IntegerDigits[n]], 1]]]; If[ PrimeQ[q] && q - p > d, Print[q]; d = q - p; p = q], {n, 2, 3000002}]
CROSSREFS
KEYWORD
base,nonn
AUTHOR
Robert G. Wilson v, Dec 03 2002
EXTENSIONS
Corrected by T. D. Noe, Oct 25 2006
Corrected and more terms from Robert Israel, Jan 31 2019
STATUS
approved