This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
a(2)=5 because the 4 consecutive primes 5, 7, 11, 13 have gaps 2, 4, 2, which is symmetric about its center 4 = 2*2.
with(numtheory):nn:=500000:l:=2:T:=array(1..2*l-1)):
for n from 1 to 35 do:ii:=0:
for k from 1 to nn while(ii=0) do:
lst:={}:lst1:={}:
for m from 1 to 2*l do:
lst:=lst union {ithprime(k+m-1)}
od:
for p from 1 to 2*l do:
lst1:=lst1 union {lst[p]+lst[2*l-p+1]}
od:
n0:=nops(lst1):
if n0=1
then
for a from 1 to 2*l-1 do:
T[a]:=lst[a+1]-lst[a]:
od:
if T[2]=2*n then ii:=1:printf(`%d, `,lst[1]):
else fi :fi:
od :
od:
a(n) = {pa = 3; pb = 5; pc = 7; forprime(p=8, , if (((pc-pb) == 2*n) && ((pb-pa) == (p-pc)), return(pa)); pa = pb; pb = pc; pc = p;);} \\ Michel Marcus, Oct 16 2015
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