A077369 - OEIS

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A077369

Smallest n-digit triangular number whose external digits as well as internal digits form triangular numbers, or -1 if no such number exists.

0, 10, 105, 1035, 19900, 162165, 1057785, 10628355, 497086215, 1037196285, 10379451160, 100611783910, 1009356921705, 22222221111111, 159874280406855, 2222222211111111, 30089723753897506, 138961477283832105, 1001626196023188555, 22222222221111111111, 200026159385335864701, 1000091594147464659855

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,2

COMMENTS

Smallest n-digit number in A077368. - Andrew Howroyd, Sep 21 2024

LINKS

Table of n, a(n) for n=1..22.

FORMULA

a(n) <= A077370(n). - Andrew Howroyd, Sep 21 2024

PROG

(PARI) \\ here isok(k) tests if k in A077368.

isok(k)={if(k<10, 1, my(b=10^logint(k, 10)); ispolygonal(k%b\10, 3) && ispolygonal(k\b*10+k%10, 3))}

a(n)={for(k=(sqrtint(8*(10^(n-1)-1)+1)+1)\2, (sqrtint(8*(10^n-1)+1) -1)\2, my(t=k*(k+1)/2); if(isok(t), return(t))); oo} \\ Andrew Howroyd, Sep 21 2024

CROSSREFS

Cf. A077366, A077367, A077368, A077370.

Sequence in context: A210136 A068093 A260214 * A068618 A096484 A190955

Adjacent sequences: A077366 A077367 A077368 * A077370 A077371 A077372

KEYWORD

base,easy,nonn

AUTHOR

Amarnath Murthy, Nov 06 2002

EXTENSIONS

Offset corrected and a(6)-a(22) from Andrew Howroyd, Sep 21 2024

STATUS

approved