

A071194


Length (>1) of shortest sequence of consecutive primes starting with prime(n) whose sum is also prime, or 1 if no such sequence exists.


7



2, 9, 3, 3, 3, 5, 3, 3, 3, 3, 3, 9, 3, 5, 7, 3, 5, 3, 3, 3, 5, 3, 3, 7, 7, 3, 7, 5, 3, 5, 5, 9, 5, 3, 3, 5, 3, 3, 11, 9, 5, 21, 5, 9, 3, 9, 3, 5, 55, 3, 7, 27, 9, 27, 7, 5, 5, 3, 9, 3, 3, 3, 5, 3, 7, 7, 11, 3, 3, 3, 5, 5, 7, 7, 3, 5, 3, 9, 3, 3, 5, 11, 3, 5, 47, 5, 3, 3, 5, 3, 3, 5, 7, 3, 3, 7, 3, 5, 5, 5, 3
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OFFSET

1,1


LINKS



EXAMPLE

For n=1, startprime = prime(1) = 2, 2+3=5 is prime, length=2, so a(1)=2;
for n=2, startprime = prime(2) = 3, 3 + 5 + 7 + 11 + 13 + 17 + 19 + 23 + 29 = 127 is prime, length=9, all shorter partial sums are composite, so a(2)=9;
for n=160, prime(160) = 941, 941 + ... + 1609 = 121123 is prime, a(160)=95.


MATHEMATICA

Table[k = 2; While[CompositeQ@ Total@ Prime@ Range[n, n + k], k++]; k + 2 Boole[EvenQ@ k]  1, {n, 120}] (* Michael De Vlieger, Jan 01 2017 *)


PROG

(PARI) a(n, p=prime(n))=my(q=p, t=2); while(!isprime(p+=q=nextprime(q+1)), t++); t


CROSSREFS



KEYWORD

nonn


AUTHOR



EXTENSIONS



STATUS

approved



