A078274 a(n) is the number from which if odd-positioned digits are deleted then one gets the concatenation of first n odd numbers, while if even-positioned digits are deleted then one gets reverse concatenation of first n odd numbers. Position of least significant digit is considered to be 1.

11, 1331, 153351, 17355371, 1937557391, 11315977951311, 113351719917151331, 1135517391111917351351, 11375175911311113917551371, 113951779115111331115917751391, 1231517991171115311351117917952311, 12335271911911173115511371119927152331

Offset: 1

Author

Amarnath Murthy, Nov 25 2002

Keywords

• base
• nonn

Comments

a(n) is formed by interleaving the digits of A019519(n) and A038395(n). - Sean A. Irvine, Jun 26 2025

Examples

			a(4) = 17355371: deleting alternate digits starting from the LSD gives 1357. Deleting the other digits gives 7531.
		

Crossrefs

Cf. A078275, A078276, A078277, A078278.

Cf. A019519, A038395.

Programs

PARI

a(n) = {my(d, v=w=[1]); for(i=2, n, v=concat(v, d=digits(2*i-1)); w=concat(d, w)); fromdigits(vector(2*#v, i, if(i%2, v[1+i\2], w[i/2]))); } \\ Sean A. Irvine, Jun 26 2025

Extensions

More terms from Antonio G. Astudillo (afg_astudillo(AT)lycos.com), Apr 19 2003

Corrected and extended by Harvey P. Dale, Apr 11 2012

Original terms restored by Sean A. Irvine, Jun 26 2025