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A237885
a(n) is the number of ways that 4n can be written as p+q (p>q) with p, q, (p-q)/2, 4n-(p-q)/2 all prime numbers.
3
0, 0, 0, 1, 1, 1, 0, 1, 1, 1, 0, 2, 0, 0, 2, 0, 0, 1, 0, 0, 2, 0, 0, 0, 1, 0, 1, 1, 0, 4, 0, 0, 2, 0, 1, 1, 0, 1, 2, 0, 0, 2, 0, 0, 3, 0, 0, 2, 0, 1, 1, 0, 0, 2, 0, 0, 1, 0, 0, 5, 0, 0, 3, 0, 0, 1, 0, 0, 0, 0, 0, 4, 0, 0, 3, 0, 0, 2, 0, 1, 3, 0, 0, 3, 1, 0, 3
OFFSET
1,12
COMMENTS
2n=q+(p-q)/2; 6n=p+(4n-(p-q)/2).
Number of ways that 2*n can be written as a+b with a<b and a, b, a+2*b and 2*a+b all prime. - Robert Israel, Jun 07 2022
EXAMPLE
When n=4, 4n=16, 16=13+3, (13-3)/2=5, 16-5=11, all four numbers {3, 5, 11, 13} are prime numbers. There is no other such four number set with this property, so a(4)=1;
When n=30, 4n=120.
120=113+7, (113-7)/2=53, 120-53=67. Set 1: {7, 53, 67, 113}.
120=109+11, (109-11)/2=49=7*7, X
120=107+13, (107-13)/2=47, 120-47=73. Set 2: {13, 47, 73, 107}.
120=103+17, (103-17)/2=43, 120-43=77=7*11, X
120=101+19, (101-19)/2=41, 120-41=79. Set 3: {19, 41, 79, 101}.
120=97+23, (97-23)/2=37, 120-37=83. Set 4: {23, 37, 83, 97}.
120=89+31, (89-31)/2=29, 120-29=91=7*13, X
120=83+37, same with Set 4.
120=79+41, same with Set 3.
120=73+47, same with Set 2.
120=67+53, same with Set 1.
120=61+59, (61-59)/2=1, X
So four acceptable sets have been found, and therefore a(30)=4.
MAPLE
N:= 100: # for a(1)..a(N)
V:= Vector(N):
P:= select(isprime, [seq(i, i=3..2*N, 2)]):
nP:= nops(P):
for i from 1 to nP do
p:= P[i];
for j from i+1 to nP do
q:= P[j];
if p+q > 2*N then break fi;
r:= (p+q)/2;
if isprime(p+2*q) and isprime(2*p+q) then
V[r]:= V[r]+1
fi
od
od:
convert(V, list); # Robert Israel, Jun 08 2022
MATHEMATICA
Table[qn = 4*n; p = 2*n - 1; ct = 0; While[p = NextPrime[p]; p < qn, q = qn - p; If[PrimeQ[q] && PrimeQ[(p - q)/2] && PrimeQ[qn - (p - q)/2], ct++]]; ct/2, {n, 1, 87}]4*n-1
PROG
(PARI) a(n)=my(s); forprime(p=2, n, if(isprime(2*n-p) && isprime(2*n+p) && isprime(4*n-p), s++)); s \\ Charles R Greathouse IV, Mar 15 2015
CROSSREFS
Sequence in context: A016424 A337985 A108913 * A341775 A139032 A182035
KEYWORD
nonn,easy
AUTHOR
Lei Zhou, Feb 14 2014
STATUS
approved