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

1,2

Comments

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

Links

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

Example

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

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

Prog

(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))

Crossrefs

Cf. A297645, A297646, A297647.

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

Sequence in context: A009413 A009429 A207821 * A242043 A156236 A209481

Adjacent sequences: A297641 A297642 A297643 * A297645 A297646 A297647

Keyword

nonn,base

Author

Colin Barker, Jan 02 2018

Status

approved