A047981 a(n) = A047980(2n).
3, 7, 17, 71, 19, 59, 167, 757, 197, 159, 799, 227, 317, 415, 361, 521, 3289, 2633, 1637, 1861, 1691, 1997, 2053, 4097, 6437, 5731, 9199, 11603, 5641, 3833, 26885, 6637, 26815, 32117, 18637, 29933, 31667, 5227, 19891, 47303, 54973, 5207, 59537
Offset: 1
Keywords
Examples
First example: a(1)=3 since in 3k+1 sequence, the first term is 3, a prime and the d=2 is the smallest such difference. The next such progression is 5k+1 because 5*2+1=11 is prime. 2nd example: here at n=6 a(6)=59. This means that 2n=12 occurs first in A034693 at its position 59, which means that its first prime is 12*59+1=709. arises as 12th term (such progressions are: 59k+1,85k+1,133k+1, etc.)
Formula
a(n) = min {k}: A034693(a(n)) is an even number such that in a(n)*k+1 progression the first prime occurs at even 2n=k position.