A156208 - OEIS

%I #19 Sep 08 2016 03:59:30

%S 2,3,5,7,3,5,7,11,13,17,19,2,3,5,7,11,13,17,19,5,7,11,13,17,19,23,7,

%T 11,13,17,19,23,11,13,17,19,23,7,13,19,3,5,11,17,23,29,7,13,19,31,11,

%U 17,23,29,13,19,31,37,17,23,29,41,19,31,37,43,2,5,11,17,23,29,7,13,19,31,11

%N Primes appearing as the products of digits and positions in A156207(i) in the order of appearance.

%C A156207 read without the 1's and without the composites. - _R. J. Mathar_, Sep 07 2016

%H Robert Israel, <a href="/A156208/b156208.txt">Table of n, a(n) for n = 1..10000</a>

%F Given a number n with digits d1d2d3...dm, a(n) = d1*1+d2*2+d3*3+...+dm*m.

%F If a(n) is prime, list it.

%e For n=19 we have 1*1 + 2*9 = 19 prime and the sequence.

%p f:= proc(n) local L,i,a;

%p   L:= convert(n,base,10);

%p   a:= add(L[-i]*i,i=1..nops(L));

%p   if isprime(a) then a else NULL fi

%p end proc:

%p map(f, [$1..1000]); # _Robert Israel_, Sep 07 2016

%o (PARI) g1(n) = for(j=1,n,if(isprime(g(j)),print1(g(j)",")))

%o g(n) = v=Vec((Str(n)));sum(x=1,length(v),x*eval(v[x]))

%Y Cf. A156207.

%K base,nonn,less

%O 1,1

%A _Cino Hilliard_, Feb 05 2009, Feb 08 2009

%E Definition clarified. - _R. J. Mathar_, Sep 07 2016