OFFSET
1,2
COMMENTS
It appears that all terms are squares. - Michel Marcus, Apr 02 2017
All terms are odd squares, because k and d(k) must be odd. - Robert Israel, Apr 04 2017
LINKS
Robert Israel, Table of n, a(n) for n = 1..10000
EXAMPLE
d(121) = 3 and 1213 and 3121 are both prime.
MAPLE
with(numtheory): P:=proc(q) local a, b, n; for n from 1 by 2 to q do
a:=n*10^(ilog10(tau(n))+1)+tau(n); b:=tau(n)*10^(ilog10(n)+1)+n;
if isprime(a) and isprime(b) then print(n); fi; od; end: P(10^9);
# Alternative:
filter:= proc(k) local d; d:= numtheory:-tau(k); isprime(d + 10^(1+ilog10(d))*k) and isprime(k+10^(1+ilog10(k))*d) end proc:
select(filter, [seq(k^2, k=1..10^4, 2)]); # Robert Israel, Apr 04 2017
MATHEMATICA
cnc[x_, y_] := FromDigits[Join @@ IntegerDigits@{x, y}]; Select[ Range[1, 5000, 2]^2, (t = DivisorSigma[0, #]; PrimeQ@ cnc[t, #] && PrimeQ@ cnc[#, t]) &] (* Giovanni Resta, Apr 04 2017 *)
PROG
(PARI) is(k)={my(v1=digits(k), v2=digits(numdiv(k))); isprime(fromdigits(concat(v1, v2)))&&isprime(fromdigits(concat(v2, v1)))};
findFirstTerms(q)={my(ans:list, k=1); ans=List(); q=max(1, q); for(c=1, q, while(!is(k), k++); listput(ans, k); k++); Vec(ans)} \\ R. J. Cano, Mar 31 2017
CROSSREFS
KEYWORD
nonn,base
AUTHOR
Paolo P. Lava, Mar 31 2017
STATUS
approved