A303263 Indices of primes in tribonacci sequence A000073.

4, 6, 7, 11, 87, 98, 215, 802, 4202, 18699, 96879

Offset: 1

Author

M. F. Hasler, Apr 18 2018

Keywords

• nonn
• hard
• more

Comments

T = A000073 is defined by T(n+1) = T(n) + T(n-1) + T(n-2), T(2) = 1, T(1) = T(0) = 0.

The largest terms correspond to unproven probable primes T(a(n)).

Crossrefs

Cf. A000073 (= T), A092836 (= T(a(n))), A092835 (= a(n) - 1).

Cf. A001605 (indices of primes in Fibonacci numbers A000045).

Programs

Mathematica

-1 + Position[LinearRecurrence[{1, 1, 1}, {0, 0, 1}, 10^4], ?PrimeQ][[All, 1]] (* _Michael De Vlieger, Apr 21 2018 *)

PARI

a(n,N=4,S=vector(N,i,i>N-2))={for(i=N,oo,ispseudoprime(S[i%N+1]=2*S[(i-1)%N+1]-S[i%N+1])&&!n--&&return(i))}

Formula

a(n) = A092835(n) + 1 = index of A092836(n) in A000073.