Esta no es una respuesta (no sé la prueba formal), pero un comentario porque el poder de Euler-sumación de series como esto es muy impresionante, pero a menudo no es realmente conocido.
Aquí hay una tabla de la progresión del valor final sin y con Euler-suma. De Euler-suma puede tener "órdenes", que intuitivamente significa, itera (pero puede ser interpolada fraccional de pedidos). Aquí está la tabla de uso de Euler-suma "a la orden (0.5)" :
the individual partial distance to partial sums distance to pi/4
terms of the series sums pi/4 by Euler-summ.
-----------------------------------------------------------------------------------------------
1.00000000000 1.00000000000 0.214601836603 1.00000000000 0.214601836603
-0.333333333333 0.666666666667 -0.118731496731 0.777777777778 -0.00762038561967
0.200000000000 0.866666666667 0.0812685032692 0.792592592593 0.00719442919514
-0.142857142857 0.723809523810 -0.0615886395879 0.784832451499 -0.000565711898330
0.111111111111 0.834920634921 0.0495224715232 0.785851459926 0.000453296528086
-0.0909090909091 0.744011544012 -0.0413866193859 0.785350269301 -0.0000478940965617
0.0769230769231 0.820934620935 0.0355364575372 0.785432796132 0.0000346327349363
-0.0666666666667 0.754267954268 -0.0311302091295 0.785393836971 -0.00000432642610791
0.0588235294118 0.813091483680 0.0276933202823 0.785401073569 0.00000291017167712
-0.0526315789474 0.760459904732 -0.0249382586651 0.785397756972 -0.000000406425094661
0.0476190476190 0.808078952351 0.0226807889539 0.785398422239 0.000000258841333253
-0.0434782608696 0.764600691482 -0.0207974719156 0.785398124198 -0.0000000391999284656
0.0400000000000 0.804600691482 0.0192025280844 0.785398187306 0.0000000239087486163
-0.0370370370370 0.767563654445 -0.0178345089527 0.785398159544 -0.00000000385353035100
0.0344827586207 0.802046413065 0.0166482496680 0.785398165666 0.00000000226867304697
-0.0322580645161 0.769788348549 -0.0156098148481 0.785398163013 -0.000000000384322878997
0.0303030303030 0.800091378852 0.0146932154549 0.785398163617 2.19652484372E-10
-0.0285714285714 0.771519950281 -0.0138782131165 0.785398163359 -3.87659406085E-11
0.0270270270270 0.798546977308 0.0131488139105 0.785398163419 2.16018278683E-11
-0.0256410256410 0.772905951667 -0.0124922117305 0.785398163394 -3.94612015099E-12
0.0243902439024 0.797296195569 0.0118980321720 0.785398163400 2.15112513145E-12
-0.0232558139535 0.774040381616 -0.0113577817815 0.785398163397 -4.04724379093E-13
0.0222222222222 0.796262603838 0.0108644404407 0.785398163398 2.16404904436E-13
-0.0212765957447 0.774986008093 -0.0104121553040 0.785398163397 -4.17725596145E-14
0.0204081632653 0.795394171359 0.00999600796131 0.785398163397 2.19558323752E-14
-0.0196078431373 0.775786328222 -0.00961183517595 0.785398163397 -4.33468728127E-15
0.0188679245283 0.794654252750 0.00925608935236 0.785398163397 2.24358184761E-15
-0.0181818181818 0.776472434568 -0.00892572882946 0.785398163397 -4.51894496246E-16
0.0175438596491 0.794016294217 0.00861813081966 0.785398163397 2.30671631889E-16
-0.0169491525424 0.777067141675 -0.00833102172271 0.785398163397 -4.73009122777E-17
0.0163934426230 0.793460584298 0.00806242090024 0.785398163397 2.38422950230E-17
-0.0158730158730 0.777587568425 -0.00781059497278 0.785398163397 -4.96870530081E-18
Tenga en cuenta que este "orden de 0.5" parece ser óptima; el simple Euler-suma (que eran "de orden 1") acelera no tan espectaculares.