

A081235


Smallest prime starting a sequence of 2n consecutive primes with symmetrical gaps about the center.


11



2, 5, 5, 17, 13, 137, 8021749, 1071065111, 1613902553, 1797595814863, 633925574060671, 22930603692243271
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OFFSET

1,1


LINKS



FORMULA



EXAMPLE

The first term is 2 since the 2 primes 2, 3 have a gap of 1, which is trivially symmetric about its center.
The second term is 5 since the 4 primes 5, 7, 11, 13 have gaps 2, 4, 2, which is symmetric about its center.
The twelve primes 137, 139, 149, 151, 157, 163, 167, 173, 179, 181, 191, 193 have gaps 2, 10, 2, 6, 6, 4, 6, 6, 2, 10, 2  symmetric about the middle, so a(6) = 137.


PROG

(PARI) A081235(n) = { my(last=vector(n*=2, i, prime(i)), m, i=Mod(n2, n)); forprime(p=last[n], default(primelimit), m=last[1+lift(i+2)]+last[1+lift(i++)]=p; for(j=1, n\2, last[1+lift(ij)]+last[1+lift(i+j+1)]==mnext(2)); return(last[1+lift(i+1)]))} \\ M. F. Hasler, Apr 02 2010


CROSSREFS



KEYWORD

more,nonn


AUTHOR



EXTENSIONS

a(12) from an anonymous participant of the project, added by Natalia Makarova, Jul 16 2015


STATUS

approved



