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A155879 a(0) = 4; for n > 0, a(n) is the smallest composite number c > a(n-1) such that c - n is also composite. 1
4, 9, 10, 12, 14, 15, 16, 21, 22, 24, 25, 26, 27, 28, 30, 33, 34, 35, 36, 39, 40, 42, 44, 45, 46, 49, 50, 51, 52, 54, 55, 56, 57, 58, 60, 62, 63, 64, 65, 66, 68, 69, 70, 75, 76, 77, 78, 80, 81, 82, 84, 85, 86, 87, 88, 90, 91, 92, 93, 94, 95, 96, 98, 99, 100, 104, 105, 106, 108 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,1

LINKS

Table of n, a(n) for n=0..68.

EXAMPLE

a(0) = 4. Subtracting n = 0 from a(0) gives 4-0 = 4, which is a composite number; subtracting n = 1 from a(1) gives 9-1 = 8, which is a composite number; subtracting n = 2 from a(2) gives 12-2 = 10, which is a composite number; subtracting n = 3 from a(3) gives 15-3 = 12, which is a composite number; etc.

MAPLE

isA002808 := proc(n) option remember; RETURN(n>= 4 and not isprime(n)) ; end: A155879:= proc(n) option remember; local a; if n = 0 then 4; else for a from procname(n-1)+1 do if isA002808(a) and isA002808(a-n) then RETURN(a) ; fi; od: fi; end: seq(A155879(n), n=0..100) ; # R. J. Mathar, Jan 31 2009

PROG

(Python)

from sympy import isprime

def composite(n): return n >= 4 and not isprime(n)

def aupton(nn):

  alst = [4]

  for n in range(1, nn+1):

    an = max(alst[-1] + 1, n + 4)

    while not (composite(an) and composite(an-n)): an += 1

    alst.append(an)

  return alst

print(aupton(68)) # Michael S. Branicky, Apr 09 2021

CROSSREFS

Cf. A155875.

Sequence in context: A175308 A244533 A180149 * A172192 A243194 A342393

Adjacent sequences:  A155876 A155877 A155878 * A155880 A155881 A155882

KEYWORD

base,easy,nonn,changed

AUTHOR

Eric Angelini, Jan 29 2009

EXTENSIONS

Corrected from a(2) on by R. J. Mathar, Jan 31 2009

Name edited by Jon E. Schoenfield, Jan 19 2019

STATUS

approved

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Last modified April 21 17:36 EDT 2021. Contains 343156 sequences. (Running on oeis4.)