A045973 a(1)=10; if n = Product p_i^e_i, n > 1, then a(n) = Product p_{i+1}^e_i * Product p_{i+3}^e_i.

10, 21, 55, 441, 91, 1155, 187, 9261, 3025, 1911, 247, 24255, 391, 3927, 5005, 194481, 551, 63525, 713, 40131, 10285, 5187, 1073, 509355, 8281, 8211, 166375, 82467, 1271, 105105, 1591, 4084101, 13585, 11571, 17017, 1334025, 1927, 14973, 21505, 842751

Offset

1,1

References

From a puzzle proposed by Marc LeBrun.

Links

Table of n, a(n) for n=1..40.

Formula

Sum_{n>=1} 1/a(n) = -9/10 + Product_{k>=1} (1+1/(prime(k)*prime(k+4)-1)) = 0.2602421684... . - Amiram Eldar, Sep 19 2023

Mathematica

f[p_, e_] := NextPrime[p]^e * NextPrime[p, 3]^e; a[1] = 10; a[n_] := Times @@ f @@@ FactorInteger[n]; Array[a, 100] (* Amiram Eldar, Sep 19 2023 *)

Crossrefs

Cf. A045965, A045966, A045967, A045968, A045969, A045970, A045971, A045972.

Sequence in context: A090076 A231966 A156592 * A095679 A095192 A356761

Adjacent sequences: A045970 A045971 A045972 * A045974 A045975 A045976

Keyword

easy,nonn

Author

N. J. A. Sloane

Extensions

More terms from David W. Wilson

Status

approved