A045969 a(1)=6; if n = Product p_i^e_i, n>1, then a(n) = Product p_{i+1}^e_i * Product p_{i+2}^e_i.

6, 15, 35, 225, 77, 525, 143, 3375, 1225, 1155, 221, 7875, 323, 2145, 2695, 50625, 437, 18375, 667, 17325, 5005, 3315, 899, 118125, 5929, 4845, 42875, 32175, 1147, 40425, 1517, 759375, 7735, 6555, 11011, 275625, 1763, 10005, 11305, 259875, 2021, 75075

Offset

1,1

References

From a puzzle proposed by Marc LeBrun.

Links

Table of n, a(n) for n=1..42.

Formula

Sum_{n>=1} 1/a(n) = -5/6 + Product_{k>=2} (1+1/(prime(k)*prime(k+1)-1)) = 0.31383788... . - Amiram Eldar, Sep 19 2023

Mathematica

f[p_, e_] := (NextPrime[p] * NextPrime[p, 2])^e; a[1] = 6; a[n_] := Times @@ f @@@ FactorInteger[n]; Array[a, 100] (* Amiram Eldar, Sep 19 2023 *)

Crossrefs

Cf. A045965, A045966, A045967, A045968, A045970, A045971, A045972, A045973.

Sequence in context: A379232 A274320 A099620 * A100513 A378746 A338057

Adjacent sequences: A045966 A045967 A045968 * A045970 A045971 A045972

Keyword

easy,nonn

Author

N. J. A. Sloane

Extensions

More terms from David W. Wilson

Status

approved