A066867 Numbers n such that 2^n has 7 as its fourth decimal digit from the right.

21, 24, 27, 32, 40, 46, 56, 62, 73, 85, 94, 141, 157, 164, 170, 175, 183, 188, 216, 228, 234, 237, 261, 265, 268, 293, 300, 317, 331, 339, 349, 355, 359, 369, 376, 379, 386, 403, 410, 430, 442, 447, 451, 454, 458, 463, 472, 495, 498

Offset

1,1

Comments

A sequence of no importance apart from the reference, which attributes the solution of this to John von Neumann, beating a computer to the solution.

References

Sylvia Nasar, A Beautiful Mind (1998), p. 80.

Links

David A. Corneth, Table of n, a(n) for n = 1..10000

Example

32 is in the sequence as 2^32 = 4294967296 which has a 7 as the fourth decimal digit from the right. - David A. Corneth, Jun 21 2022

Mathematica

Select[ Range[ 10, 500 ], IntegerDigits[ 2^# ][ [ -4 ] ] == 7 & ]
Select[Range[500], NumberDigit[2^#, 3]==7&] (* Harvey P. Dale, Jun 21 2022 *)

Prog

(PARI) is(n) = lift(Mod(2, 10000)^n) \ 1000 == 7 \\ David A. Corneth, Jun 21 2022
(Python)
def ok(n): return pow(2, n, 10000)//1000 == 7
print([k for k in range(500) if ok(k)]) # Michael S. Branicky, Jun 21 2022 after David A. Corneth

Crossrefs

Cf. A068345.

Sequence in context: A303313 A257642 A332922 * A111356 A344806 A295692

Adjacent sequences: A066864 A066865 A066866 * A066868 A066869 A066870

Keyword

nonn,easy,base

Author

Harvey P. Dale, Jan 21 2002

Status

approved