A297644 Pentagonal numbers (A000326) in which parity of digits alternates.

1, 5, 12, 70, 92, 145, 210, 852, 925, 2147, 2501, 3290, 3432, 3876, 4187, 4347, 6305, 6501, 12105, 12927, 25676, 27270, 27676, 45850, 58707, 69230, 69876, 70525, 76501, 78547, 98945, 101270, 123410, 161212, 270725, 349692, 367290, 567030, 707610, 709672

Offset: 1

Author

Colin Barker, Jan 02 2018

Keywords

• nonn
• base

Comments

Intersection of A000326 and A030141. - Felix Fröhlich, Jan 03 2018

Examples

			3876 is in the sequence because 3, 8, 7 and 6 have odd and even parity alternately.
		

Links

• Robert Israel, Table of n, a(n) for n = 1..2000

Crossrefs

Cf. A297645, A297646, A297647.

Cf. A000326, A030141, A030152, A030160, A068882.

Programs

Maple

filter:= proc(n) local L;
  if n < 10 then return true fi;
  L:= convert(n, base, 10) mod 2;
  not has(L[2..-1]-L[1..-2], 0)
end proc:
select(filter, [seq(n*(3*n-1)/2, n=1..1000)]); # Robert Israel, Jan 03 2018

PARI

is_alt(n) = m=n; e=n%10; n\=10; while(n>0, f=n%10; if(e%2==f%2, return, e=f; n\=10)); return(m)
select(is_alt, vector(1000, n, (3*n^2-n)/2))