login
This site is supported by donations to The OEIS Foundation.

 

Logo

Please make a donation to keep the OEIS running. We are now in our 55th year. In the past year we added 12000 new sequences and reached 8000 citations (which often say "discovered thanks to the OEIS"). We need to raise money to hire someone to manage submissions, which would reduce the load on our editors and speed up editing.
Other ways to donate

Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A136358 Increasing sequence obtained by union of two sequences {b(n)} and {c(n)}, where b(n) is the smallest odd composite number m such that both m-2 and m+2 are prime and the set of distinct prime factors of m consists of the first n odd primes and c(n) is the smallest composite number m such that both m-1 and m+1 are primes and the set of the distinct prime factors of m consists of the first n primes. 6
4, 6, 9, 15, 30, 105, 420, 2310, 3465, 15015, 180180, 765765, 4084080, 106696590, 247342095, 892371480, 3011753745, 9704539845, 100280245065, 103515091680, 4412330782860, 29682952539240, 634473110526255, 22514519501013540 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

This sequence is different from A070826 and A118750.

LINKS

Table of n, a(n) for n=1..24.

EXAMPLE

a(1)=4 is preceded by 3 and followed by 5, both primes; a(3)=9, preceded by 7 and followed by 11, both primes.

MATHEMATICA

b[n_]:=(d=Product[Prime[k], {k, n}]; For[m=1, !(!PrimeQ[d*m]&&PrimeQ[d*m-1] &&PrimeQ[d*m+1]&&Length[FactorInteger[c*m]]==n), m++ ]; d*m); c[n_]:=(d=Product [Prime[k], {k, 2, n+1}]; For[m=1, !(!PrimeQ[d*(2*m-1)]&&PrimeQ[d(2m-1)-2]&&PrimeQ [d(2m-1)+2]&&Length[FactorInteger[d(2m-1)]]==n), m++ ]; d(2m-1)); Take[Union[Table [b[k], {k, 24}], Table[c[k], {k, 24}]], 24] (* Farideh Firoozbakht, Aug 13 2009 *)

PROG

(UBASIC)

10 'A136358, Enoch Haga, Jun 19 2009'

11 'compute and combine input 2 or 3 separately; begin with 4 and 9

20 input "prime, 2 or 3"; A

30 if A=2 or A=3 then B=nxtprm(A)

40 print A; B; :R=A*B:print R; :stop

50 if even(R)=1 then if R-1=prmdiv(R-1) and R+1=prmdiv(R+1) then print "*"

60 if even(R)=0 then if R-2=prmdiv(R-2) and R+2=prmdiv(R+2) then print "+"

61 print R:stop

70 B=nxtprm(B):R=B*R

90 print B; R:stop

100 goto 50

- Enoch Haga, Jul 11 2009

CROSSREFS

Cf. A136349-A136357, A070826, A118750.

Sequence in context: A065856 A136357 A136356 * A115665 A118694 A085648

Adjacent sequences:  A136355 A136356 A136357 * A136359 A136360 A136361

KEYWORD

easy,nonn

AUTHOR

Enoch Haga, Dec 25 2007

EXTENSIONS

Edited, corrected and extended by Farideh Firoozbakht, Aug 13 2009

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified December 9 22:27 EST 2019. Contains 329880 sequences. (Running on oeis4.)