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 A088426 Number of primes in arithmetic progression starting with 19 and with d=2n. 10
 1, 2, 1, 1, 2, 3, 1, 1, 2, 1, 2, 3, 1, 2, 1, 1, 2, 1, 1, 2, 3, 1, 1, 2, 1, 2, 4, 1, 1, 4, 1, 2, 1, 1, 2, 1, 1, 1, 2, 1, 2, 2, 1, 2, 3, 1, 2, 1, 1, 1, 1, 1, 1, 2, 1, 2, 1, 1, 2, 2, 1, 1, 1, 1, 2, 3, 1, 1, 2, 1, 1, 3, 1, 2, 1, 1, 2, 1, 1, 2, 2, 1, 1, 1, 1, 2, 4, 1, 2, 3, 1, 1, 1, 1, 1, 2, 1, 1, 1, 1 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Arithmetic progression is stopped when next term is not prime. E.g. for n=6 (d=12), a=3, that is 19,31,43 are prime, while next term, 55, is not prime. From Robert Israel, Jul 27 2020: (Start) a(n) = 1 if n == 1 (mod 3), a(n) <= 2 if n == 2 (mod 3). If a(n) >= p where p is 3, 5, 7, 11, 13 or 17, then n is divisible by p. All a(n) < 19. Records: a(1)=1 a(2)=2 a(6)=3 a(27)=4 a(210)=5 a(825)=6 a(16380)=7 a(273420)=9 a(17853675)=10 (End) From David A. Corneth, Jul 29 2020: (Start) Other first occurences are: a(779520) = 8 a(4918073160) = 11 a(3187366788375) = 12 a(6125952702870) = 13 If a(k) = 14 then k > 4.8*10^15. If a(k) = 15 then k > 1.77 * 10^16. (End) LINKS Robert Israel, Table of n, a(n) for n = 1..10000 MAPLE f:= proc(n) local d, k;   d:= 2*n;   for k from 1 while isprime(19+d*k) do od:   k end proc: map(f, [\$1..200]); # Robert Israel, Jul 27 2020 MATHEMATICA bb={}; Do[s=1; Do[If[PrimeQ[19+k*d], s=s+1, bb={bb, s}; Break[]], {k, 10}], {d, 2, 200, 2}]; Flatten[bb] CROSSREFS Cf. A005115, A088420, A088421, A088422, A088423, A088424, A088425, A088427, A088428, A088429. Sequence in context: A333765 A088198 A318288 * A124769 A176484 A144328 Adjacent sequences:  A088423 A088424 A088425 * A088427 A088428 A088429 KEYWORD easy,nonn AUTHOR Zak Seidov, Sep 29 2003 STATUS approved

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Last modified May 16 01:55 EDT 2021. Contains 343937 sequences. (Running on oeis4.)