A109920 a(1) = 1, then LCM of consecutive composite numbers sandwiched between primes.

1, 2, 1, 3, 4, 5, 6, 7, 360, 11, 12, 13, 1680, 17, 18, 19, 4620, 23, 491400, 29, 30, 31, 1884960, 37, 29640, 41, 42, 43, 45540, 47, 12994800, 53, 45821160, 59, 60, 61, 89369280, 67, 164220, 71, 72, 73, 211110900, 79, 265680, 83, 195878760, 89

Offset: 1

Author

Amarnath Murthy, Jul 16 2005

Keywords

• easy
• nonn

Crossrefs

Cf. A109919.

Programs

Maple

A109920 := proc(n) local p; if n mod 2 = 0 then ithprime(n/2) ; elif n = 1 then 1 ; else p := ithprime((n-1)/2) ; lcm(seq(i,i=p+1..nextprime(p)-1)) ; fi ; end: for n from 1 to 80 do printf("%d, ",A109920(n)) ; od ; # R. J. Mathar, May 02 2007

Formula

a(2n) = prime(n) a(2n+1)= LCM of composite numbers between prime(n) and prime(n+1). a(1) = a(3) = 1 by choice.

Extensions

More terms from R. J. Mathar, May 02 2007