cp's OEIS Frontend

A158215 Smallest palindromic prime made up of 0's and p(n) 1's, where p(n) is the n-th prime = A000040(n) (or 0 when no such prime exists).

%I A158215 #10 Nov 06 2015 02:01:26
%S A158215 11,0,100111001,110111011,1110111110111,10111101110111101,
%T A158215 100111111111111111001,1111111111111111111,11111111111111111111111,
%U A158215 1111110111111111111111110111111,11111101111111110101111111110111111
%N A158215 Smallest palindromic prime made up of 0's and p(n) 1's, where p(n) is the n-th prime = A000040(n) (or 0 when no such prime exists).
%C A158215 Smallest palindromic prime with digit sum A000040(n) and using only 0's and 1's. Subsequence of A158214.
%C A158215 Smallest palindromic prime with digit sum A000040(n) and using only 0's and 1's. Subsequence of A100580.(Sequence link edited). [_Lekraj Beedassy_, Jun 21 2009]
%H A158215 Chai Wah Wu, <a href="/A158215/b158215.txt">Table of n, a(n) for n = 1..168</a>
%o A158215 (Python)
%o A158215 from __future__ import division
%o A158215 from itertools import combinations
%o A158215 from sympy import prime, isprime
%o A158215 def A158215(n):
%o A158215     if n == 1:
%o A158215         return 11
%o A158215     if n == 2:
%o A158215         return 0
%o A158215     p2 = prime(n)//2
%o A158215     l = p2
%o A158215     while True:
%o A158215         for i in combinations(range(l),l-p2):
%o A158215             s = ['1']*l
%o A158215             for x in i:
%o A158215                 s[x] = '0'
%o A158215             s = ''.join(s)
%o A158215             q = int(s+'1'+s[::-1])
%o A158215             if isprime(q):
%o A158215                 return q
%o A158215         l += 1 # _Chai Wah Wu_, Nov 05 2015
%Y A158215 Cf. A157712
%Y A158215 Cf. A158214. [_Lekraj Beedassy_, Jun 21 2009]
%K A158215 nonn,base
%O A158215 1,1
%A A158215 _Lekraj Beedassy_, Mar 13 2009