A390771 a(n) is the minimal concatenation pq of two primes p <= q such that p + q = 2*n.

22, 33, 35, 37, 57, 77, 313, 513, 317, 319, 519, 323, 523, 723, 329, 331, 531, 731, 337, 537, 341, 343, 543, 347, 547, 747, 353, 553, 753, 359, 361, 561, 761, 367, 567, 371, 373, 573, 773, 379, 579, 383, 583, 783, 389, 589, 789, 1979, 397, 597, 797, 1789, 1197

Offset

2,1

Links

Michael S. Branicky, Table of n, a(n) for n = 2..10001

James S. DeArmon, R Code for A390771

Example

a(2) = 22 because 2+2 = 4.
a(3) = 33 because 3+3 = 6.
a(8) = 313 because 3+13 = 16.

Maple

a:= n-> min(seq((l-> `if`(andmap(isprime, l),
      parse(cat(l[])), [][]))([n-i, n+i]), i=0..n-2)):
seq(a(n), n=2..60);  # Alois P. Heinz, Nov 28 2025

Prog

(R) # See DeArmon link.
(Python)
from sympy import isprime, sieve
def a(n): return min(int(str(p)+str(q)) for p in sieve.primerange(3, n+1) if isprime(q:=2*n-p)) if n > 2 else 22
print([a(n) for n in range(2, 55)]) # Michael S. Branicky, Dec 01 2025

Crossrefs

Cf. A073046, A014091, A157931, A000040.

Sequence in context: A167337 A216798 A100372 * A084996 A250180 A252078

Adjacent sequences: A390768 A390769 A390770 * A390772 A390773 A390774

Keyword

nonn,base

Author

James S. DeArmon, Nov 18 2025

Status

approved