A354367 Successive pairs of terms (a, b) such that (a + b) is a square and at least one of a and b is a prime number. This is the lexicographically earliest infinite sequence of distinct terms > 0 with this property.

1, 3, 2, 7, 4, 5, 6, 19, 8, 17, 10, 71, 11, 14, 12, 13, 15, 181, 18, 31, 20, 29, 21, 43, 22, 59, 23, 26, 24, 97, 27, 37, 28, 53, 30, 139, 32, 89, 33, 67, 34, 47, 35, 109, 38, 83, 39, 61, 40, 41, 42, 79, 44, 317, 45, 151, 46, 179, 48, 73, 50, 239, 51, 349, 52, 173, 54, 307, 55, 269, 56, 113, 57, 199, 58

Offset

1,2

Comments

This is not a permutation of the integers > 0 as no square > 4 will appear. Two prime terms can form a pair (2 and 7 for instance) but at least one term must be prime [the pair (1, 3) is ok].

Links

Table of n, a(n) for n=1..75.

Michael De Vlieger, Annotated log-log scatterplot of a(n), n = 1..10^4, showing records in red, numbers entering late in blue, highlighting primes in green, fixed points in gold, and composite prime powers in magenta.

Example

The earliest pairs with their square sum: (1, 3) = 4, (2, 7) = 9, (4, 5) = 9, (6, 19) = 25, (8, 17) = 25, (10, 71) = 81, (11, 14) = 25, (12, 13) = 25, etc.

Mathematica

nn = 10^4; c[_] = 0; a[1] = c[1] = 1; u = 2; Do[k = u; If[EvenQ[i], While[Nand[c[k] == 0, AnyTrue[{#, k}, PrimeQ], IntegerQ@ Sqrt[# + k]] &[a[i - 1]], k++]]; Set[{a[i], c[k]}, {k, i}]; If[k == u, While[Or[c[u] > 0, And[IntegerQ@ Sqrt@ u, u > 4]], u++]], {i, 2, nn}]; Array[a, nn] (* Michael De Vlieger, May 24 2022 *)

Crossrefs

Cf. A354368, A354369, A354370 (same idea).

Sequence in context: A175057 A153154 A154438 * A194071 A194104 A277679

Adjacent sequences: A354364 A354365 A354366 * A354368 A354369 A354370

Keyword

nonn

Author

Eric Angelini and Carole Dubois, May 24 2022

Status

approved