A258742 - OEIS

A258742

With a(1) = 1, a(n) is the smallest positive number not already in the sequence such that a(n)^2 + a(n-1)^2 is not prime.

1, 3, 4, 2, 6, 7, 5, 9, 8, 10, 11, 12, 14, 13, 15, 16, 17, 19, 18, 20, 21, 22, 24, 23, 25, 27, 26, 28, 29, 31, 32, 30, 33, 34, 36, 37, 35, 38, 39, 41, 40, 42, 44, 43, 45, 46, 47, 49, 48, 50, 52, 51, 53, 54, 55, 56, 57, 58, 59, 60, 62, 61, 63, 64, 65, 67, 66, 68, 69, 71, 72, 70, 73, 74, 75, 76, 77, 78, 79, 81

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OFFSET

1,2

COMMENTS

Believed to be a permutation of the natural numbers.

LINKS

Table of n, a(n) for n=1..80.

MATHEMATICA

f[n_] := Block[{a = {1}, k}, For[k = 2, k <= n, k++, i = 1; While[Or[PrimeQ[i^2 + a[[k - 1]]^2], MemberQ[a, i]], i++]; AppendTo[a, i]]; a]; f@ 120 (* Michael De Vlieger, Jun 10 2015 *)

PROG

(PARI) v=[1]; n=1; while(n<100, if(!isprime(n^2+v[#v]^2)&&!vecsearch(vecsort(v), n), v=concat(v, n); n=0); n++); v

CROSSREFS

Cf. A055266, A100208.

Sequence in context: A049277 A214917 A260316 * A372862 A143052 A297104

Adjacent sequences: A258739 A258740 A258741 * A258743 A258744 A258745

KEYWORD

nonn

AUTHOR

Derek Orr, Jun 08 2015

STATUS

approved