

A155879


a(0) = 4; for n > 0, a(n) is the smallest composite number c > a(n1) 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
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OFFSET

0,1


LINKS

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


EXAMPLE

a(0) = 4. Subtracting n = 0 from a(0) gives 40 = 4, which is a composite number; subtracting n = 1 from a(1) gives 91 = 8, which is a composite number; subtracting n = 2 from a(2) gives 122 = 10, which is a composite number; subtracting n = 3 from a(3) gives 153 = 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(n1)+1 do if isA002808(a) and isA002808(an) 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(ann)): 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



