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A300951 a(n) = Product_{j=1..floor(n/2)} p(j) where p(j) = j if j is prime else 1. 1
1, 1, 1, 1, 2, 2, 6, 6, 6, 6, 30, 30, 30, 30, 210, 210, 210, 210, 210, 210, 210, 210, 2310, 2310, 2310, 2310, 30030, 30030, 30030, 30030, 30030, 30030, 30030, 30030, 510510, 510510, 510510, 510510, 9699690, 9699690, 9699690, 9699690, 9699690, 9699690, 9699690 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,5

COMMENTS

a(4*n+2)=a(4*n+3)=a(4*n+4)=a(4*n+5) for n >= 1. - Robert Israel, Mar 16 2018

The length of the n-th run is given by 2*A054541(n). - Michel Marcus, Mar 17 2018

LINKS

Robert Israel, Table of n, a(n) for n = 0..4713

FORMULA

a(n) = A002110(A056172(n)). - Robert Israel, Mar 16 2018

MAPLE

a := n -> mul(`if`(isprime(j), j, 1), j=1..iquo(n, 2)):

seq(a(n), n=0..44);

# Alternative:

f:= proc(n) option remember;

  if n::even and isprime(n/2) then procname(n-1)*n/2 else procname(n-1) fi

end proc:

f(0):= 1:

map(f, [$0..100]); # Robert Israel, Mar 16 2018

PROG

(PARI) a(n) = prod(i=1, n\2, if(isprime(i), i, 1)); \\ Altug Alkan, Mar 16 2018

CROSSREFS

Cf. A002110, A054541, A056172, A089026.

Sequence in context: A319865 A140219 A259225 * A077081 A084700 A122766

Adjacent sequences:  A300948 A300949 A300950 * A300952 A300953 A300954

KEYWORD

nonn

AUTHOR

Peter Luschny, Mar 16 2018

STATUS

approved

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Last modified October 22 07:48 EDT 2019. Contains 328315 sequences. (Running on oeis4.)