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A196938
a(n) is the prime number that is the center element (3rd) of a 5-terms arithmetic progression prime chain.
2
17, 29, 67, 71, 89, 97, 101, 131, 163, 167, 173, 191, 193, 197, 211, 233, 241, 257, 263, 269, 283, 307, 313, 317, 337, 347, 373, 419, 433, 443, 449, 457, 461, 463, 467, 479, 491, 503, 509, 521, 523, 547, 577, 599, 601, 607, 617, 619, 631, 641, 643, 677, 683
OFFSET
1,1
COMMENTS
The Mathematica program gives the first 53 terms.
The Mathematica program is also good for finding sequences with any odd number of terms.
EXAMPLE
{5,13,[17],23,29} is a 5-term arithmetic progression prime chain, so a(1)=17; (for all primes smaller than 17, no such chains exist)
{5,17,[29].41,53} is a 5-term arithmetic progression prime chain, so a(2)=29; (for all primes in between 17 and 29, no such chains exist)
MATHEMATICA
terms = 5; max=53; i = 1; step = (terms - 1)/2; Table[While[i++; p = Prime[i]; found = 0; j = 0; While[j++; df = 6*j; (p > (step*df)) && (found == 0), found = 1; Do[If[(! PrimeQ[p - k*df]) || (! PrimeQ[p + k*df]), found = 0], {k, 1, step}]]; found == 0]; p, {ct, 1, max}]
CROSSREFS
Cf. A033451.
Sequence in context: A089629 A007349 A154616 * A225943 A069755 A076727
KEYWORD
nonn,easy
AUTHOR
Lei Zhou, Oct 07 2011
STATUS
approved