A354376 Smallest prime which is at the end of an arithmetic progression of exactly n primes.

2, 3, 7, 43, 29, 157, 907, 2351, 5179, 2089, 375607, 262897, 725663, 36850999, 173471351, 198793279, 4827507229, 17010526363, 83547839407, 572945039351, 6269243827111

Offset

1,1

Comments

Equivalently: Let i, i+d, i+2d, ..., i+(n-1)d be an arithmetic progression of exactly n primes; choose the one which minimizes the last term; then a(n) = last term i+(n-1)d.

The word "exactly" requires both i-d and i+n*d to be nonprime; without "exactly", we get A005115.

For the corresponding values of the first term, and the common difference, see A354377 and A354484. For the actual arithmetic progressions, see A354485.

The primes in these arithmetic progressions need not be consecutive. (The smallest prime at the start of a run of exactly n consecutive primes in arithmetic progression is A006560(n).)

a(n) != A005115(n), because A005115(n) + A093364(n) is prime for n = 4, 8, 9, 11. - Michael S. Branicky, May 24 2022

References

R. K. Guy, Unsolved Problems in Number Theory, A5, Arithmetic progressions of primes.

Links

Table of n, a(n) for n=1..21.

Example

The arithmetic progression (5, 11, 17, 23) with common difference 6 contains 4 primes, but 29 = 23+6 is also prime, hence a(4) != 23.
The arithmetic progression (7, 19, 31, 43) with common difference 12 also contains 4 primes, and 7-12 < 0 and 43+12 = 55 is composite; moreover this arithmetic progression is the smallest such progression with exactly 4 primes, hence a(4) = 43.

Prog

(Python)
from sympy import isprime, nextprime
def a(n):
    if n < 3: return [2, 3][n-1]
    p = 2
    while True:
        for d in range(2, (p-3)//(n-1)+1, 2):
            if isprime(p+d) or isprime(p-n*d): continue
            if all(isprime(p-j*d) for j in range(1, n)): return p
        p = nextprime(p)
print([a(n) for n in range(1, 11)]) # Michael S. Branicky, May 24 2022

Crossrefs

Cf. A005115, A006560, A093364, A354377, A354484, A354485.

Sequence in context: A261564 A126263 A323605 * A216826 A030087 A106864

Adjacent sequences: A354373 A354374 A354375 * A354377 A354378 A354379

Keyword

nonn,more

Author

Bernard Schott, May 24 2022

Extensions

a(4) corrected and a(8)-a(13) from Michael S. Branicky, May 24 2022

a(14)-a(21) derived using A005115 and A093364 by Michael S. Branicky, May 24 2022

Status

approved