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a(n) is the number of natural numbers k such that A078496(k)=n.
0

%I #6 Nov 06 2019 17:50:13

%S 1,0,2,0,0,0,3,0,2,0,0,0,3,0,3,0,0,0,3,0,0,0,0,0,3,0,5,0,0,0,0,0,3,0,

%T 0,0,5,0,4,0,0,0,3,0,0,0,0,0,4,0,0,0,0,0,3,0,6,0,0,0,0,0,4,0,0,0,5,0,

%U 4,0,0,0,0,0,4,0,0,0,5,0,0,0,0,0,4,0

%N a(n) is the number of natural numbers k such that A078496(k)=n.

%F a(n) = #{k: k>3 integer; A078496(k)=n}.

%e a(19)=3 since there are only k=13,16,18 for which A078496(k)=19

%o (PARI)

%o b(n)={forprime(p=nextprime(n+1), 2*n, if(isprime(2*n-p), return(p))); 0}

%o a(n)={if(isprime(n), sum(i=n\2+1, n-1, b(i)==n), 0)} \\ _Andrew Howroyd_, Nov 06 2019

%Y Cf. A078496.

%K easy,nonn

%O 5,3

%A Serhat Sevki Dincer (sevki(AT)ug.bilkent.edu.tr), Apr 18 2003

%E Terms a(24) and beyond from _Andrew Howroyd_, Nov 06 2019